Where does mass go?

Thanks to Ardaozkal his comment I was able to do some research.

There are a couple of reasons why the game never runs out of space,

• When just going around not collecting any orbs you seem to lose a bit off mass. The amount of mass you lose seems to be bigger if you have a lot.
• When pressing W you shoot out a bit off mass, picking it back up will not give you back the amount you lost.

Splitting up or hitting a spike does not seem to make you lose anything other then your shape.