I finally understood my issue :
I was using a lot of different pictures as textures, without respecting the “power of 2” rule.
So my texture were not able to be streamed, and they were asking a huge resource size.
Here is a link to a related topic : https://answers.unrealengine.com/questions/549411/cant-untick-never-stream.html
So my solution is to re-import all my images as png, in a 2048x2048 square, using the alpha channel to keep the original picture ratio.