An average game (fps for instance) basically needs to receive player x is at 123, 2, 87, facing at angle 43, angle of elevation -11, and is holding gun x.
Starmade needs to receive: Planet is here, planet is made of (info describing what all the blocks are). Station is here, planet is made of (info describing what all the blocks are). Ship is at position, attitude, and is made of these blocks. Pilot is blasting away at planet with 2000 single block cannons, all the projectile information. Someone is placing blocks in symmetry mode 8000 at a time, data on all the placed blocks....
This is oversimplified, but having to continuously download the maps and ships requires a lot of bandwidth. However it should be a lot lower than what you have said when you are sitting still in a fully loaded sector with nobody else around.