How is a doggie life jacket different than a human one?

So i've been letting my 80lbs pit bull wear my human Small Female (kayaking) life jacket when we go to the blue holes because he's kind of a goofy swimmer. I've been looking at doggie life jackets, but they seem alot cheaper quality than the one he wears now, and basically I was wondering if there was any danger in just letting him wear mine or perhaps just ordering him a used womens life jacket since they seem like they fit his chest and legs well?