diff --git a/manim/_config/utils.py b/manim/_config/utils.py
index 3e45846539..da93865b1f 100644
--- a/manim/_config/utils.py
+++ b/manim/_config/utils.py
@@ -327,7 +327,7 @@ class MyScene(Scene): ...
     }
 
     def __init__(self) -> None:
-        self._d: dict[str, Any | None] = dict.fromkeys(self._OPTS)
+        self._d: dict[str, Any] = dict.fromkeys(self._OPTS)
 
     # behave like a dict
     def __iter__(self) -> Iterator[str]:
@@ -423,13 +423,13 @@ def __copy__(self) -> Self:
         """See ManimConfig.copy()."""
         return copy.deepcopy(self)
 
-    def __deepcopy__(self, memo: dict[str, Any]) -> Self:
+    def __deepcopy__(self, memo: dict[int, Any]) -> Self:
         """See ManimConfig.copy()."""
         c = type(self)()
         # Deepcopying the underlying dict is enough because all properties
         # either read directly from it or compute their value on the fly from
         # values read directly from it.
-        c._d = copy.deepcopy(self._d, memo)  # type: ignore[arg-type]
+        c._d = copy.deepcopy(self._d, memo)
         return c
 
     # helper type-checking methods
@@ -1142,24 +1142,22 @@ def frame_width(self, value: float) -> None:
     @property
     def frame_y_radius(self) -> float:
         """Half the frame height (no flag)."""
-        return self._d["frame_height"] / 2  # type: ignore[operator]
+        return self._d["frame_height"] / 2
 
     @frame_y_radius.setter
     def frame_y_radius(self, value: float) -> None:
-        self._d.__setitem__("frame_y_radius", value) or self._d.__setitem__(  # type: ignore[func-returns-value]
-            "frame_height", 2 * value
-        )
+        self._d["frame_y_radius"] = value
+        self._d["frame_height"] = 2 * value
 
     @property
     def frame_x_radius(self) -> float:
         """Half the frame width (no flag)."""
-        return self._d["frame_width"] / 2  # type: ignore[operator]
+        return self._d["frame_width"] / 2
 
     @frame_x_radius.setter
     def frame_x_radius(self, value: float) -> None:
-        self._d.__setitem__("frame_x_radius", value) or self._d.__setitem__(  # type: ignore[func-returns-value]
-            "frame_width", 2 * value
-        )
+        self._d["frame_x_radius"] = value
+        self._d["frame_width"] = 2 * value
 
     @property
     def top(self) -> Vector3D:
@@ -1290,9 +1288,8 @@ def frame_size(self) -> tuple[int, int]:
 
     @frame_size.setter
     def frame_size(self, value: tuple[int, int]) -> None:
-        self._d.__setitem__("pixel_width", value[0]) or self._d.__setitem__(  # type: ignore[func-returns-value]
-            "pixel_height", value[1]
-        )
+        self._d["pixel_width"] = value[0]
+        self._d["pixel_height"] = value[1]
 
     @property
     def quality(self) -> str | None:
diff --git a/manim/mobject/_fit.py b/manim/mobject/_fit.py
new file mode 100644
index 0000000000..1e77bed86e
--- /dev/null
+++ b/manim/mobject/_fit.py
@@ -0,0 +1,51 @@
+"""Fit-to-size operations for :class:`~.Mobject`.
+
+Standalone functions that take a mobject as their first argument; the
+corresponding methods on :class:`Mobject` are thin delegations to these
+helpers. This keeps :class:`Mobject`'s public API unchanged while moving
+the implementation out of the already-large ``mobject.py``.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from manim.mobject.mobject import Mobject
+
+
+def rescale_to_fit(
+    mob: Mobject, length: float, dim: int, stretch: bool = False, **kwargs: Any
+) -> Mobject:
+    old_length = mob.length_over_dim(dim)
+    if old_length == 0:
+        return mob
+    if stretch:
+        mob.stretch(length / old_length, dim, **kwargs)
+    else:
+        mob.scale(length / old_length, **kwargs)
+    return mob
+
+
+def scale_to_fit_width(mob: Mobject, width: float, **kwargs: Any) -> Mobject:
+    return rescale_to_fit(mob, width, 0, stretch=False, **kwargs)
+
+
+def stretch_to_fit_width(mob: Mobject, width: float, **kwargs: Any) -> Mobject:
+    return rescale_to_fit(mob, width, 0, stretch=True, **kwargs)
+
+
+def scale_to_fit_height(mob: Mobject, height: float, **kwargs: Any) -> Mobject:
+    return rescale_to_fit(mob, height, 1, stretch=False, **kwargs)
+
+
+def stretch_to_fit_height(mob: Mobject, height: float, **kwargs: Any) -> Mobject:
+    return rescale_to_fit(mob, height, 1, stretch=True, **kwargs)
+
+
+def scale_to_fit_depth(mob: Mobject, depth: float, **kwargs: Any) -> Mobject:
+    return rescale_to_fit(mob, depth, 2, stretch=False, **kwargs)
+
+
+def stretch_to_fit_depth(mob: Mobject, depth: float, **kwargs: Any) -> Mobject:
+    return rescale_to_fit(mob, depth, 2, stretch=True, **kwargs)
diff --git a/manim/mobject/mobject.py b/manim/mobject/mobject.py
index 721ce57356..c635f06e50 100644
--- a/manim/mobject/mobject.py
+++ b/manim/mobject/mobject.py
@@ -22,6 +22,7 @@
 import numpy as np
 
 from manim.data_structures import MethodWithArgs
+from manim.mobject import _fit
 from manim.mobject.opengl.opengl_compatibility import ConvertToOpenGL
 
 from .. import config, logger
@@ -1752,13 +1753,7 @@ def stretch_about_point(self, factor: float, dim: int, point: Point3DLike) -> Se
     def rescale_to_fit(
         self, length: float, dim: int, stretch: bool = False, **kwargs: Any
     ) -> Self:
-        old_length = self.length_over_dim(dim)
-        if old_length == 0:
-            return self
-        if stretch:
-            self.stretch(length / old_length, dim, **kwargs)
-        else:
-            self.scale(length / old_length, **kwargs)
+        _fit.rescale_to_fit(self, length, dim, stretch=stretch, **kwargs)
         return self
 
     def scale_to_fit_width(self, width: float, **kwargs: Any) -> Self:
@@ -1784,7 +1779,8 @@ def scale_to_fit_width(self, width: float, **kwargs: Any) -> Self:
             >>> sq.height
             np.float64(5.0)
         """
-        return self.rescale_to_fit(width, 0, stretch=False, **kwargs)
+        _fit.scale_to_fit_width(self, width, **kwargs)
+        return self
 
     def stretch_to_fit_width(self, width: float, **kwargs: Any) -> Self:
         """Stretches the :class:`~.Mobject` to fit a width, not keeping height/depth proportional.
@@ -1809,7 +1805,8 @@ def stretch_to_fit_width(self, width: float, **kwargs: Any) -> Self:
             >>> sq.height
             np.float64(2.0)
         """
-        return self.rescale_to_fit(width, 0, stretch=True, **kwargs)
+        _fit.stretch_to_fit_width(self, width, **kwargs)
+        return self
 
     def scale_to_fit_height(self, height: float, **kwargs: Any) -> Self:
         """Scales the :class:`~.Mobject` to fit a height while keeping width/depth proportional.
@@ -1834,7 +1831,8 @@ def scale_to_fit_height(self, height: float, **kwargs: Any) -> Self:
             >>> sq.width
             np.float64(5.0)
         """
-        return self.rescale_to_fit(height, 1, stretch=False, **kwargs)
+        _fit.scale_to_fit_height(self, height, **kwargs)
+        return self
 
     def stretch_to_fit_height(self, height: float, **kwargs: Any) -> Self:
         """Stretches the :class:`~.Mobject` to fit a height, not keeping width/depth proportional.
@@ -1859,15 +1857,18 @@ def stretch_to_fit_height(self, height: float, **kwargs: Any) -> Self:
             >>> sq.width
             np.float64(2.0)
         """
-        return self.rescale_to_fit(height, 1, stretch=True, **kwargs)
+        _fit.stretch_to_fit_height(self, height, **kwargs)
+        return self
 
     def scale_to_fit_depth(self, depth: float, **kwargs: Any) -> Self:
         """Scales the :class:`~.Mobject` to fit a depth while keeping width/height proportional."""
-        return self.rescale_to_fit(depth, 2, stretch=False, **kwargs)
+        _fit.scale_to_fit_depth(self, depth, **kwargs)
+        return self
 
     def stretch_to_fit_depth(self, depth: float, **kwargs: Any) -> Self:
         """Stretches the :class:`~.Mobject` to fit a depth, not keeping width/height proportional."""
-        return self.rescale_to_fit(depth, 2, stretch=True, **kwargs)
+        _fit.stretch_to_fit_depth(self, depth, **kwargs)
+        return self
 
     def set_coord(
         self, value: float, dim: int, direction: Vector3DLike = ORIGIN