additional arguments and java heap

When I start the deployed app without specifying Xms & Xmx memory 
range, it starts fine.  However, if I specify these args it does not.  
Would anyone know why this happens?

I am using args -Xms=256 -Xmx=1024, and earlier I got it work.  Later, 
I bumped it up to -Xmx=2048 (or 4096 not sure) which is when I started 
having this problem.

Thanks

--Brian