I think the type of mirror has a sh*t ton to do with it. It depends on the exact position/angle of the mirror, the surrounding light/front/rear, quality of mirror, etc. I don't notice a big difference between pics and mirrors, but mostly between mirrors themselves if that makes sense.
For instance, if you have a bathroom mirror, and the only lights are above it, if you're right on top of it you'll look less defined. Now step back 8-10 feet and look. You should appear more ripped again just due to the change in shadowing/lighting against your body.