Did you try to open a completely new map and see if this also appears in an empty map? If this appears in every map of your project maybe somewhere you are using either very many textures or some gigantic textures. If this is the case then your empty map should be just fine.
I would also guess that this issue is not related to the FBX models themselves or it’s size, because they are just combinations of vertices and UV-maps, aren’t they? They have nothing to do with textures in the first place. They can have default materials and those materials use textures.
Are you using some materials/textures always or in every map? If so, then check those out and maybe you can locate the source of the problem on this way =/