Launch Calamus with the [Ctrl] key pressed. You will then see the Calamus Parameters dialog (the one with the huge feather) where you can reserve RAM for Calamus. I guess you have reserved either less than 64 MB or more than 1.5 GB of RAM.
Calamus SL still is a TOS application and TOS never knew about more than 2 GB of RAM. Subtract the RAM required by the wrapper Calamus.exe, then you will see that you must not assign more than 1.5 GB RAM for Calamus.
I myself usually use only 128 MB, in rare cases 512 MB. For any fat documents, I prefer the High-Resolution Data Manager.