Texture file size changes when I export

Im importing a PNG mask map with 4k and just 100% black and 100% white and 319 kb. When I check in size map, the size goes to 11,2 mb. Whats the logic behind this (bug, maybe)?

There`s a way to solve this?

This is quite normal and to be expected since UE4 (and basically any engine) compresses the texture to their own (direct x/compression) related format, which works differently than regular .jpg, .png, etc.
This is also why you want to use a texture that is uncompressed so it doesnt get compressed twice.

so while it sucks that your filesize becomes much bigger, its just how texture compression works to make it run as good as possible.

I understand the situation. That`s kind of frustrating, I imagine there’s no way around this.
Thanks for your help!