Allie, it's becuase photobucket resizes your pics to whatever size you told it to. Size used to be dictated by your selection in a listbox right at the uploader screen. Now you have to click on 'more options' to change that.
When they're on your computer, they're whatever size you told your camera to take them. If you ahve a coolpix, then I can help you but it's probably not something you want to mess with because you'll loose resolution on that 1 in 1000 picture that you'd want as a 8x10.
When you view pictures on your computer, use windows photo gallery. That will tell you exactly what size the picture is. Then you can cross reference that to maximum allowable size here (800x600 I think, don't quote me)
Then the last piece of the puzzle is making sure your CP options are set correctly. Under 'Edit Options' Look for 'Thread Display Options', make sure 'Please select the resize mode :' says keep original size.
I was just playing around wtih it on crookedcreek's thread. If it's bigger than a thumbnail, it doesn't seem to matter what size the picture you upload is - it's always a thumbnail. If you upload it to a hosting service, it links in a viewable size.
Some of the features here are counter intuitive. Or hidden in a place where you can't find them. Duh, my mailbox could have been emptied in one simple click. It would be sooo nice if they'd put a big list of stuff that would make life easier.