Is there a way to raise the available memory to the web starter version of SH3D?

Unfortunately not without creating an other jnlp file. I wanted to request a larger memory size for 64 bit architecture like I did for Windows and Linux, but it wasn't possible under macOS because of a bug. Anyway, here's a link to another Java Web Start version that will request 4GB max but will work only under 64 bit systems.
You could also use the Jar executable version, but you would lose the file association with .sh3d files.