Representation of bromelain molecule in two dimensional diagram
I think I got it. The diagram in PubChem is not bromelain molecule, but a tiny fragment of it. This thing is indeed too small to be an enzyme. Also, it has nothing to do with proteins. Look at the formula, there is not a single amino acid residue here. Indeed, scroll down to 3.4.2 and read: "carbohydrate moiety of bromelain". That's what it is, and not the entire enzyme.

If you want to know what the rest of it is made of, then google "bromelain amino acid sequence". It seems to be known for quite a while. No nice pics, though.

On a side note, there is no such thing as "the smallest protein" or even "the smallest known protein", for the border between proteins and polypeptides is rather arbitrary. You are right, though, in that most enzymes are huge proteins, too big to fit on a 2D diagram. So is bromelain.

So it goes.