"I've been working out a lot the last few months. I managed to lose a lot of weight and I think I look good."

"Yeah, you looks great, that work definitely paid off."

The coworker brought it up, which indicates that they're comfortable talking about it and the comment is a celebration of the coworker's achievement rather than any kind of expression of sexual desire.

And then of course there are professions where the body is directly relevant to the work, like acting, modelling and sports/atheleticism.

All this really depends on context though. In a corporate office where everyone's barely more than strangers, I don't think even the situation above would be appropriate. In a small business where a "coworker" is as much a close friend as a colleague, I think the threshold for what's acceptable is very different.