Hmm...  Photoshop is reading a wee bit more than 3 gigs of RAM out of my 4. (Bryce reads 2 no matter what..I checked the CPU usage when it was rendering something at premium render) ..
the file size isn't that much.. 22 megs for this one pix. The other one that it did that to was about 60 megs
But you could be right with using big brushes. Patience.. I need to get a cup of coffee or a choc bar then.. sigh....