Hi Romanito,
What platform are you publishing to? If you are developing for mobile platforms, your meshes greatly exceed the max amount of triangles as described here in our documentation: (Roughly 64K)
https://docs.unrealengine.com/latest/INT/Platforms/Mobile/Content/index.html
As for 1.0 to 1.5 million tris for static meshes, this is virtually unheard of in games due to the effect on rendering frames per second. Typically models of that size are used to produce normal maps that are then applied to lower poly models for the illusion of great detail.
To give you an idea of polycounts used in AAA titles, a main character runs between 65K-100K max.
As for the difference in sizes between the ctm, fbx, and uasset file sizes. The ctm is raw triangle data with some UV data as opposed to the uasset that contains applied texture, collision, pivot point, -and a lot of other data- that makes it game compatible. The fbx falls somewhere in between.
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