Reader Sgt.LulzJager has kept the same female Commander Shepard model across all three Mass Effect games. That's a lot of time spent with the same character. So it didn't take long for him to notice something different about her in Mass Effect 3.
Beside a few tweaks to her face, he didn't make any changes to the rest of her physical appearance. That all just...happened.
I noticed my Shepard's arms had gotten bigger, but figured, hey, he's been in what's essentially a prison for a while. And dudes in prison pump iron.
I may not watch enough documentaries on women's prisons to be an authority on this subject, but I'm pretty sure women in the slammer don't come out with complimentary cosmetic surgery.