Total filesize of textures doubling after build.

My entire assets folder is 208 MB. When I build the project, editor.log reports:

"Textures 398.7 mb 96.9% "

"Complete size 411.6 mb 100.0% "

Why is the final build so large and what can I do to reduce it?

What’s doubling? 411.6MB is less than 1% of 208GB.