SWTOR managed to do this. I managed to create the thinnest Sith and the fattest Trooper, so I don't see why they can't in ME. It's not important for me, but choice is always good I guess. Btw, I'd never call M!Shep beefy, not like james.
Assuming varying body shapes/sizes are ever introduced, it should reflect the background of the character being portrayed.
An obese Shepard for example, wouldn't have made sense given his/her background as a military officer. The character has a background where some level of physical fitness is part of the requirements for getting the job and exercise is part of the day to day activities of that job. In order not to demolish suspension of disbelief, with that same background any introduction of varying body shapes in the CC would have to be limited from thin and fit to muscular and fit.
That sort of thing is less an issue of course if your character is a pirate or some merchant captain.