Yeah, welcome to optimization 
The biped class needs to the parent class to exist, hence that.
I’m afraid it’s a very large topic.
As you can see, textures are a big issue ( always ). You can reduce the mip level for a lot of them, maybe. You won’t see a difference in the size map, but they package with the lower resolution.
If you want to see it changing ‘live’ in the size map, you’ll need to actually change the size of them. Then you’ll need a plugin, like