I manually verified the size of your server and it is correct.
The displayed file size is compressed, uncompressed your server is 5.8G big - 5.0G of that is your world.
Problem is solved now. (Thank you)
But, it was 4.08 Gb before. And we downloaded the world and cleared unused chunks and reuploaded, then server's size was 4.6 Gb. May be my friend tried to upload world again and this caused to server's size grow to 5.8 Gb.
But last time i uploaded world, it replaced old world. But this time it didn't... I think someone from aternos admins manually solved the problem.