I wonder what is the equivalence of saying “7 feet tall” or “1 inch thick” for weight. Is it “55 pounds heavy”?
That would make sense -- which, since this is English we're talking about here, naturally means that it's wrong. Instead the idiomatic expression is:
55 pounds in weight
Again, naturally, this is completely redundant, because what else would "pounds" measure other than weight? Nevertheless, this is what native speakers actually say.
(OK, sure, "pounds" might be UK money, but in most contexts it would be obvious which you were talking about. Still, that may be why the idiom persists, since it is possible to be ambiguous:)
Jim: I lost 10 pounds!
Henry: Congratulations! Good for you.
Jim: No, I mean I lost a tenner when I was riding the Tube. It must have fallen out of my pocket.