You sure that the size of the 400D pics is larger than the 350D pics?
If so, it might be that ps tries to open at a scale, that allows the complete pic to show. And that may be a smaller scale with a 10MP photo than with the 8MP if the latter just fit the workspace/window in ps.
That would then be the same as with Paintshop.
Well let me re-write it,, i have photoshop CS2 not paintshop but i also have paintshop pro XI on CD, ok when i open a picture from my 400D in photoshop the picture is small here's what the size say's on left hand bottom 16.67%, Doc: 28.8m/28.8m, now when i open a picture from the 350D i used to own, the picture is bigger here's what the size say's 25%, Doc: 22.8m/22.8m, is there anyway so when i open my picture from 400D to have the same size on screen thank you.