it's not rude. is it?

is it rude to say someone looks like an animal? i mean as long as the animal is not like really ugly or fat or something it's okay right? today i told my friend his legs looked like a sandpiper. he laughed it off for a bit then got all mad.... i don't understand that.... he laughed cuz he knew it was true then got really offended. it;s not like i called his 5' 6'' 100lbs an elephant. i called him a small bird cuz his legs are skinny and he knows it. *sign* people are so sensitive over stupid stuff. :p'