It really has nothing to do with XPression. Photoshop has always been a huge memory hog on systems. I've worked for several other graphics manufacturers in the past, and we always discouraged customers from running Photoshop on their real-time graphics systems, if possible.
If you have to run it on an XPression, be sure you exit Photoshop before doing anything on air. Ideally, finish your work, exit the software, and reboot the machine, because even after closing the application it still seems to hang onto some resources.
#XPression