The Beta 10 released this weekend fixes some bugs related to unit conversion in Virtual visitor and Compass dialog boxes, and attributes 2 GB of max memory to Sweet Home 3D run with Java Web Start under 64 bit Windows and Linux systems. Sorry, I didn't find a way to increase the max memory under Mac OS X 64 bit too (sounds like a bug that appears only under that system), but if needed, there's always the solution to use installer version and set the max memory. Under Windows, the JRE launched by default seems to be the 32 bit version if you installed both 16 and 32 bit JREs.
This max memory now appears in the About dialog box for information.