Usually what jacks the file size up is the key frames. Even a high mesh model doesn't do as much to bloat a file as thousands of keyframes might. These can come from various places: a complex and long camera track can do it to you, using the set keyframes button on a multi object model can set the keyframes (needed or not) down through the object stack really multiplying the number of keyframes in the scene.
In the model category, baked deformation subD info can really add up the memory cost. As can baked or internally stored cache info for say fabric sims.
But don't discount those textures - while they are stored externally and don't add to the file size, when rendering, those all have to fit into memory to do their job efficiently.
Having more parametric objects can also add to the size. C4D has to keep all the elements down through the parametric chain sorted so they can remain "live" and changeable, that takes more memory (usually) than just having the final object - a bool of 50 objects is bigger in ram than the final collapsed editable object. Baking some of this in or making these objects editable can reduce that.
Mograph and TP can also jack the size depending on how you are using it.