I'd say 80% of the time, Photoshop opens my reasonably sized JPGs at 0.132%. I would love for them to appear at 100% or 75% or some normal size. So that I don't have to adjust it every time.
A quick google search for "default zoom photoshop" shows that there is no way to set default zoom within photoshop, however, many users have experienced problems similar to yours.
A synopsis of the posts located here: http://www.photoshopforums.com/photoshop-default-zoom-has-changed-vt21963.html
One can perform a workspace reset prior to beginning operations, clearing previous configurations.
Their instructions are that while starting Photoshop, you immediately hold down: ctrl+alt+shift (Windows) or Command+Option+Shift (Mac OS).
If you have custom workspace settings you would have to reset them.
The other option suggested in the above link is a bit more obscure:
The problem occurs whenever the photoshop workspace (the grey empty area behind documents) is smaller in area than the file you are opening.
If these answers will not resolve your problem, consider a search using the above terms and select as many links as needed to find the winning answer.