I'm presuming you know the size of a .22
The bullet is the same diameter, but it's longer and heavier than the .22 rimfire, in all in it's various versions: short, long, long-rifle, magnum, max, etc. The catridge case is much larger, which means it holds much more powder which in turn means that that heavier, longer .223 round is going much faster when it leaves the barrel. That in turn means it has much more momentum and much, much more energy. (Momentum=mass * velocity, Energy = mass * velocity * velocity / 2)