out of heap space, every day

This maybe more of a general java q than a solr one, but I'm a bit  
confused.

We have a largish solr index, about 8M documents, the data dir is  
about 70G. We're getting about 500K new docs a week, as well as about  
1 query/second.

Recently (when we crossed about the 6M threshold) resin has been  
stopping with the following:

/usr/local/resin/log/stdout.log:[12:08:21.749] [28304] HTTP/1.1 500  
Java heap space
/usr/local/resin/log/stdout.log:[12:08:21.749]  
java.lang.OutOfMemoryError: Java heap space

Only a restart of resin will get it going again, and then it'll crash  
again within 24 hours.

It's a 4GB machine and we run it with args="-J-mx2500m -J-ms2000m" We  
can't really raise this any higher on the machine.

Are there 'native' memory requirements for solr as a function of  
index size? Does a 70GB index require some minimum amount of wired  
RAM? Or is there some mis-configuration w/ resin or solr or my  
system? I don't really know Java well but it seems strange that the  
VM can't page RAM out to disk or really do something else beside  
stopping the server.