"built like a brick shithouse" is the correct phrase, its British slang and it is used to refer to a MAN who is large in terms of being muscular (rather than fat) - an example would be "I wouldn't mess with that guy, he is built like a brick shithouse". Now, whether Americans are misunderstanding this is another matter but that is what it means. I have never heard the term used in connection with a woman other than in a derogatory sense, for example "she was pig ugly and built like a brick shithouse", it does NOT mean voluptuous or curvy. So either Madonna misunderstood the meaning of the phrase or she is shading Britney, only she knows which is true.

Hope that clears a few things up for you ;o)