Answer:
First - GENERALLY, polypeptides are shorter than proteins. It's a non-specific term, but usually it's amino acid < polypeptide < protein.
But - by "equal protein" I assume you mean a protein with the same amino acid sequence and length. In that case, there is another difference. Polypeptides aren't necessarily biologically active molecules. Once they're biologically active, they're proteins, or enzymes. All you care about in a polypeptide is the amino acid sequence. But in proteins, the primary structure is the amino acid sequence, but you're also VERY concerned about secondary structure (folding & conformational changes), tertiary structure (cross bonds among amino acids in the chain) and quarternary structure - association of multiple independent polypeptides into a single protein structure.
In short - polypeptides all you really care about is the order of a long string of amino acids. With proteins, you're worried about exactly how that long string is bent, twisted, folded, and tied into other strings that have been bent, twisted and folded, to become a protein like hemoglobin that you need to breathe...