Well we can go back to the concept of beauty as per a dozen of different philosophers, but I think this is the type of stuff that can't be explained. You either feel it or you don't. I for myself cannot bypass criteria (which is I would believe hardwired into our brains) such as an inherent fondness for harmony of parts, elegance, grace and so on, the sheer animal pleasure that the eye finds in observing a well-shaped object, be it a man's body or a woman's. Maybe it's the beauty that one finds in an object that expresses its potential to the maximum - I mean, have you seen some athletes performing, like swimmers, or marathonists? People who draw might have a heightened sense of that particular beauty - the tension, the weight and how the parts move together gracefully.
This is all of course separated from any sexual context, and when sex comes into play that's where it gets tricky. I think we can draw a line that starts from your invested interest in a certain gender, as a sexual person. If you're a man and straight, you might find the male form beautiful, but not attractive. As a straight man myself, that's how it happens with me: I find the female body beautiful and I'd like to perform hella raw stuff on/with it. The male body - hella beautiful, yes (am thinking right now of some battle scenes from the movie "300", with all those seminaked Spartans), but just as with a beautiful and well-constructed chair, that's where it stops. You mentioned that a strong dude's body makes you think about a machine for lifting heavy things - isn't it possible to think of a *beautiful* machine for lifting heavy things? You can see how a guy could get confused.