I am uncertain why, but Starmade takes all of my systems available memory, despite that I have it set to only 4 GB. (Up to 5.5 GB and beyond, I only have 8 available to my system.) I assume a lot of it is unmanaged in that the Java Runtime has no control over it, hence the reason it goes over the 4 GB limit set by JVM.
OS: Arch Linux x64 (Possibly OS specific)
On a side note: I am also experiencing a SigSegV every once in a while, which could also very well be from unmanaged code too.
Edit: I have not tested Single Player, am unsure if it is affected.