OS X V10.7 Lion :: Image Editing Programs - Heavy Memory Usage?

I am wondering whether the memory use I see is common to the Mac OS X - touted as the graphics pros choice - it's seems anything but to me.  I have a mid 2010 iMac 27" 3.2 GHz Intel Core i312 GB 1333 MHz DDR3 (just upgraded in the hope that it would improve the problem - it didn't)ATI Radeon HD 5670 512 MBMac OS X Lion 10.7.4  That spec should be more than ample.  No.  Pixelmator uses up to 5GB yes - GB) just opening the app.  If I dare to open an actual image - let's say a 120mb jpg for editing - the memory usage is even higher - usually resulting in the image not opening at all.  GIMP is similar, but not as extreme - opening the file uses around 2GB.  Nevertheless, both of the above seem to me to be examples of excessive memory usage.  Certainly, my machine slows to a crawl when doing anything related to images?

Info:
Mac OS X (10.7.3), New iMac - bought Dec 2010