"I rehabilitate squirrels and chipmunks and must tell you that your 9" wheel with a 4-4.5" track is IDEAL for chipmunks (from young juvenile to large adult)... all different species. Its a must have.
The 12.5" wheels are good for young grey or fox squirrels, safe and a lot of fun, but become too small and completely loose their interest as they age (say past 9 months)... unlike chipmunks who can run in the wheels all day, squirrels quickly switch to climbing and in/out of cage acrobatics. MM#2240"