I can remember when there were some "serious" memory leaks (long long ago in the distant past years ago) when it could eat up all your memory and leave 10MB free, out of 1GB installed.
My understanding is that G5s use memory (or code can be optimized quite differently) and use more than a G4.
I was surprised that OW 5.5 was 'only' using a couple hundred MB. If you want to push it, open 100 windows, preferably jpeg's, and see what shows up.
I assume that Tiger will also do more to cache windows as well as code. Or whether each window has its own "local" code or 'thread' attached. Or whether disk cache has any affect on memory use.
It seems rather well behaved and tame in memory footprint. And what is "allocated" to VM doesn't mean it is even used (like that 1.2GB I often see owned by kernel but not written to disk or paged out).
VueScan lets you allocate xxx for previews, and yyy for scanning which seems to help, and in my case I assign 2-3x the default (VueScan use to always create multiple swap files in the past, too, but doesn't now).