I'm looking for a more colloquial / informal way to say that someone's got 'broad shoulders' or that someone's backside is 'as broad as a barn door'.
The term or phrase should not distinguish between a well-built physique and an untrained one. So it should just refer to how extremely wide someone's back is.
Let's say you are sitting in a Café and a man with a very wide back goes by your table. Amazed about that guy's appearance you turn to your partner and say something like, "Gosh, that guy's back is as broad as a barn door!"
Can you think of any other things one could say?
There is also an idiomatic meaning to broad shoulders. It means the person can carry a big burden, but a metaphorical one, not a physical one.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Oh, I mean back, not backside.