A 20 inch arm lean is very impressive
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Your true arm size shows when you actually diet down a bit to a relative level of leanness. Mine are less than 18 inches cold, and you'd think they'd be more impressive looking at 5'7, but I have long orangutan arms so they don't.
It's all about the look, dense thick forearms + thick arm + well developed delt is the trifecta (like bboy). I think most guys can grow their arms to a decent size with enough time, and patience, but the ones with cartoonish looking arms have the gift from mom and dad.