It's possible something is wrong with your virtual machine. On a clean system to run bitcoind 1GB should not cause the process to die. Check also your diskspace assigned just in case.
There is nothing wrong with my system. I have no swap (thats the main reason) and other running daemons (no clean system). Additionally there is a memory leak on bitcoind. There is no memory leak on other crypto cur. daemons like namecoind. Namecoind releases the memory it reserves.
or it could be because bitcoin's memory pool (aka transaction list) gets larger, whereas namecoin stays the same.