I cut down an old apple tree several years ago and when it resprouted from the roots I took care of it and brought it back to productivity. Last year I ended up with apples the size of grapefruits.
They aren't pretty red apples but they are very good and very sweet apples.
Apple trees are odd critters. Every seed is a different breed; you are getting apples from the root stock. This is most likely a new breed. You should patent it and sell a couple of branches to Stark Brothers. Most new breeds are crap, but if you have a good one you may be sitting on a bit of cash. Look into it.
BTW, I grew up on an apple orchard, I know a bit about this stuff. Unfortunately I didn't pay attention when my dad and uncle grafted trees; having a heck of a time figuring out how to do that!