It's possible that some people are, though the rest of the Lestrange clan was not nearly this thin. Rodolphus himself had a beater's build, just like their father. And mother would be what TV Tropes calls "Hollywood pudgy".

Only the youngest, and these days it's down to a hyper metabolism.

Rabastan turns a little pink when the waitress gives him a wink.

"Have they managed to introduce Meribia to the joys of fish and chips yet?"