Possibly it didn’t occur to him because it’s not true.
Fact is that men’s shorts tend to be a lot longer than women’s. Why do you think that is?
Because men have no fashion sense. Shorter men’s shorts were a much better invention.
“Have I mentioned lately what spectacular assets you have?”
“Clearly, your outstanding intellect and sparkling wit! Whatever did you think I meant?”
My husband’s yoga shorts really couldn’t be much shorter. They are certainly shorter than anything I wear.
I mostly see men and women wearing similar shorts. ymmv
I’m surprised at this. It seems fairly obvious that this is not typically the case. Looking around, I see some commentary as to why this is so, but it seems pretty well accepted as fact (though there has apparently been a trend to shortening men’s shorts a bit).
https://www.quora.com/Clothing-Why-are-womens-shorts-shorter-than-mens-shorts
http://www.thephoenixnews.com/2014/11/the-truth-beneath-womens-volleyball-shorts-sexualizing-womens-athletics-or-improving-performance/
[On a related note, do you also dispute that necklines for women’s shirts tend to be lower than for mens, or do you agree that there’s a difference here? And what about (dressy) sleeves?]
I would say that there is enormously more variation in the neckline of women’s shirts. I am wearing a turtleneck blouse today. Men almost never wear turtlenecks at work. But I would certainly agree that it is FAR more common for women to wear low necklines than for men to do so. And lots of women’s blouses are sleeveless, which is nearly unheard of in ordinary menswear. And typical “short sleeves” on a woman’s blouse are shorter than a men’s short sleeves.
Which is funny, because women routinely complain about being cold at work, and men almost never do. 
Except that’s only true for the last decade or two, and mostly in the U.S. at that.
So the answer is either ‘current fashion’, or “Larry Bird and Kareem Abdul-Jabbar wanted you to comment on their nice legs, wheras modern men don’t”. Which one do you think is correct?
I’ll go with “current fashion”.
What’s your point?
Would you make that comment to a man? “Hey, Jeff, nice butt today!”
Chances are, if it would be inappropriate to say to a man, it’s probably inappropriate to say to a woman. Or flip the script. If that same woman (whose boobs you are thinking about commenting on) decided that she had to comment on your junk. “Say, Bob, looks like you’re hanging to the left today! Looking good!”
While you might appreciate a compliment on your body, it’s still reductive and dehumanizing. By making that comment, you are telling her that it doesn’t really matter how she behaves or what she achieves. She’ll have your approval of her boobs. Wouldn’t you rather people focus on who you are and what you do, rather than how you look?