The amount of "dust" that is produced is quite small, and is mostly from scratching off the surfaces - we're talking about a few atoms being scrapped loose where the cutting edge scrapes into the can.
Metal deforms - like aluminum foil crumpling when you press on it - more than it crumbles. You don't really get crumbs of metal (unless a large, obvious piece breaks off) - you get a tiny sprinkle of a few atoms that get knocked off.
You can test this for yourself with a new penny and a butter knife. Scratch up a penny with a knife, and then rub the scratched-up penny on your palm for a minute. Now smell your palm - that metallic smell is the copper plating you rubbed off of the zinc coin. But you'll notice that you don't see any visible dust or flakes - because it's such a small amount of metal.