This is slang for saying a woman has big legs and/or a big butt. I guess kind of like how we use the word "thick" in the US to describe a woman built like that. It's actually a compliment, although there's no promise any young lady who hears this will take it that way. :>)
I'm not sure if this is a mexicanism or not, since both my Mexican and Colombian friend were familar with the term.
The word comes from the word "pata", which refers to the leg of animal, or an inanimate object, like a table.
Guys, this is something you only want to use around your amigos. So when you're hanging out with the guys and you see an attractive woman who meets this description, a ¡Que patotas! may well be in order.