It is the memory available to the jvm, I quoted it wrong:

ps -ax
 2040 ?        SNl   61:45 /usr/bin/java -Xms128m -Xmx856m -jar start.jar STOP.PORT=7873 STOP.KEY=0815


-Xms128m is the minimum that is reserved at the start, and -Xmx856m is the maximum it can have, in megabytes. I often had crashes with a maximum smaller 450MB, but with giving it almost all the memory, it runs quite good.

Except that dl'ing the whole blockcahin is quite some venture.