I hadn't really looked into the picture file size until you made mention to it but from what I understand go into settings and you change the resolution to 5MP. The size of the file depends on the type of picture you are taking, the surroundings, lighting, etc, etc.
I believe why the file size is small is due to the compression rate of the picture. This causes the picture to be much smaller in memory usage without suffering in quality. The Evo may not have this function or it may have an option to choose the compression rate.
In either case, I wouldn't worry about it, the picture is still 5MP but the compression rate has been changed to allow as many pictures to be stored as possible. Hope this helps.