Is this a program included with DimPro, a 3rd party expansion pack or a custom creation? It there anything else in the project. Does the program load in standalone DimPro? You may want to make sure there is no hidden error dialog on the screen. Maybe some info about your hardware and OS could be useful.
FWIW, I grabbed a large piano patch "11- Pianos Grand Pianos 3v m3 rt.prog" and loaded another large piano sfz in a second element with no difficulty. Together they well exceeded 320MB.