Never comment on a woman's rear end. Never use the words 'large' or 'size' with 'rear end.' Never. Avoid the area altogether. Trust me.
Women are like cars: we all want a Ferrari sometimes want a pickup truck...
The betrayal of trust carries a heavy taboo.
I think at its best the American sense of humor is the same as the British sense of humor at its best which is to be wry and ironic and self deprecating.