With all humility, I think I can say that I'm in pretty good shape. I finished a trail half marathon in 1:39:08 last weekend, and have several 5k age group awards. Once upon a time, I could have lifted some moderately heavy things, but that's only now relatively true. (I'm about 145 lbs.)
If this sounds like bragging, it isn't. The blogosphere is filled with more impressive models of fitness than I, both in terms of aesthetics and performance. There are those with 1:15 half marathon times, and others with 3% body fat at 200+ lbs.
It is merely a statement of fact, meant to make clear that, as far as regular folks go, I'm somewhat fit. This context is necessary, because I want it to mean something when I say that barista-ing is work. As in, it's genuinely taxing physical labor.
Or at least it can be, provided the bar is busy. Standing, moving, shuffling around behind a counter and out in front of it for 8 hours is enough to wear on even the most calloused feet.
Then there is the tamp. It varies, from bar to bar, but most advocate for at least 30 lbs of pressure, per tamp; some go upwards of 50. That, in and of itself, is not much. But neither is standing up. Either one, extrapolated out and with enough volume or frequency, can certainly be, however.
All of this is to say, I think I've solved the mystery of why baristas tend to be so thin. The job doesn't pay spectacularly well, first of all, so meals are typically infrequent, and equally insubstantial. And lunch breaks? That's a nice thought, but nothing more. Combine those factors with an 8 hour plyometrics session (hopping around behind the bar) and a zillion shoulder presses (the tamp), and you've got a best selling diet plan just waiting to be written.