I created a CC5 character with a subD2, and then clothing it with custom made cloth. I used the "hide body mesh tool" to hide the faces that are bellow the cloth (see image in attachement)

Then I export the character as a FBX for unreal engine checking the "Delete hidden Face" option.

The resulting file is huge, around 700 mb.

When I open it into blender I can see that while the faces are deleted the vertices are still there, all of them holding a ton of morph information and wasting a lot space.

Removing loose vertices in blender drops the file size to 300mb.

Also, Seems that the whole body holds morph shape keys, but they are only shape keys that affect the face.
By splitting the head as a separate geometry and deleting the shape keys on the body the file dropped to 170 mb.

The size of the fbx is crucial when you import it in Unreal as sometimes it can take up to an hour to decode the fbx (which is an unreal problem in itself IMHO). However if CC5 could perform those operations on export to fbx it would be really great.

The step I described can be reproduced by making any character in CC and clothing it then exporting as an FBX, it's not specific to my model.