It's not about size so much as price. A tablet that basically does all the things you would use an iPad for, but costs half as much, would presumably eat into Apple's sales. The idea is that you make an iPad in the same price/size range as the Kindle and Nook tablets to compete.
Eventually you either run out the competition by price-matching a product with a more popular brand name, or you eek out a tiny profit from this new niche rather than let somebody else take it.
But the thing is that the Nook and the Kindle are designed to just get you to use Amazon and B&N to buy other things. The actual hardware doesn't make much profit, it's the gateway to further purchases that makes it worth it to those companies. Apple is just the opposite, they make very little profit off of the app store but make high profits on their hardware.
So the only logical reason to do this is if they want to simply get rid of all the competitors and then eventually just one day raise the prices? I don't know.