There are three boxes at the bottom which control the memory allocation. Suggested size, Minimum size, and Preferred size. Suggested size is the amount of memory that the software manufacturer suggests you have allocated. Minimum size is the smallest segment that the application will even try to load under. If there isn't this much memory available, a message saying no memory will pop up.
Preferred size is the one to change.
This is the amount that will be allocated to the application if said amount exists. The problems arise when applications quit, but do not relinquish their allocated memory. The most common culprit is that 32-bit addressing is turned off in the Memory Control Panel.