If you're an average guy who justhappens to be tall or heavy you will lose to a trained boxer despite them being smaller. They might not possess the power to knock you out but your teeth,eyes, and nose will get wrecked and you'll likely quit after taking a few blows and landing nothing yourself. Obviously if you're a big strong dude like Shaq as someone named that might be different especially if you grapple and a mediocre trained heavyweight boxer is flatlining a contender in a lower weight class. It all is what it is.