In Spanish this means "small cactus". In American slang this a reference to a small penis. A nopalitas is a person who compensates for their lack of penis length by telling tall tales. It is a unique form of "small-man" syndrome. Can be used like a tribal name (e.g. Rising Sun) and in this case is capitalized.
He said the clerks boobs were as big as my head, but seeing that this was coming from Nopalitas, I adjusted the picture in my head down to DD cups.