Count me as one more who says you're not beefy. To me, beefy has always implied thick -- generally a muscular guy with a top layer of body fat. Or, alternately, a guy can be beefy without a top layer of body fat if he's extremely muscular, to the level of a competitive body builder. That doesn't mean that athletes can't be beefy, but think along the lines of shot putters or linemen in American football or other sports where raw strength is of central importance. I'd say you're defined, athletic, or if you want you
can actually use the term swimmer's build, as you've got a big enough chest while swill retaining a relatively low body fat percentage (it always amazes me how many thin guys pick swimmer's build. Have you looked at a competitive swimmer lately? They tend to be way more muscular than most of us thin guys are.)
I'd say this guy is beefy:
Beefy's not my cup of tea, but I doubt the guy meant it as an insult. Maybe he was trying to go for the notion that hot guys used to get called beefcakes, so logically, they would be beefy?