Most modern kitchen sink drains are this dimension--both on the garbage disposal side and the "rinse" side. These are well made items that work very effectively. Unlike most kitchen sink stoppers that rest on top of the drain opening and extend upwards into the sink when you're doing dishes, these fit down into the large opening and seal the smaller opening above the garbage disposal or "rinse" sink drain strainer. In other words they seal the drain without extending upwards into the sink full of water.
At one point, I did have a leak that I eventually determined was due to improper installation. (This was by the same "professional" plumber who reversed the hot and cold supply lines when installing a kitchen faucet. He just didn't connect the drain hose properly.) While investigating the issue, I called Waste King to inquire about possible in-home service or warranty replacement. I was blown away by their level of customer service: no waiting on hold, just an immediate connection to a person who really knew the product and was unexpectedly diligent in resolving my problem -- which, as it turns out, wasn't even their fault.
Some other kinds of garbage disposal units are powered by water pressure, rather than electricity. Instead of the turntable and grind ring described above, this alternative design has a water-powered unit with an oscillating piston with blades attached to chop the waste into fine pieces. Because of this cutting action, they can handle fibrous waste. Water-powered units take longer than electric ones for a given amount of waste and need fairly high water pressure to function properly.
Another common cause for a leaking garbage disposal is the disposal flange. A leak at the very most upper area of the garbage disposal could be the garbage disposal flange which goes through the sink. A garbage disposal flange should be sealed with Plumber’s putty and then tightened from underneath the sink. If this is not tight enough or if it has managed to come loose (which can happen) then the garbage disposal will have to be taken down so the flange can be resealed and then the garbage disposal can be rehung.