I think you are getting confused between Olympic vs standard plates rather than bumper plates vs iron.
Olympic plates are designed to fit an Olympic bar i.e. the sleeves are about 50mm diameter. The largest plates are about 450mm diameter as others have mentioned above. The difference between bumper and iron plates is that bumper plates will stay the same diameter down to at least the 10kg size so that the load is spread when dropping the bar to the floor. They are also wider to help with this as well.
On the other hand, iron plates will reduce in diameter with each reduction in weight. However they are still “Olympic” plates because of the type of bar they are used on.
Standard plates use the narrow diameter bars.