what gives?

This dwarf version is advertised to have 3/4 inch fruit on its own already....

A callery pear for example could be top grafted over to another variety of large pear which is what I would do. It is undesirable for them to make fruit aka produce seed because they are known to be invasive. They are very useful as a root stock in my location. Callery pears will pollinate many varieties of pears but I'm not sure if it will pollinate your neighbors. If It does act as a pollinator remember the neighbors tree only will get the larger fruit. Callery pear tree fruit will always be small . The next generation of callery seeds might make larger fruited unique varieties when the trees were grown from seed as shown in the picture above. Those trees in the above picture were not intentionally grown but rather grew by accident on a farm in my area. Some day work may be done to use callery to breed better pears. In the late 1800's the kieffer pear was grown from a cross between the sand pear and European pear so natural crosses do happen across species. Garber is another sand pear hybrid highly resistant to fire blight. Hybrid pears are fascinating because of their disease resistance.

The normal size range of Callery pear fruit is 1/2-5/8 in. Individuals with significantly larger fruit will then presumably be hybrids (if such crosses occur with any frequency), mutations or other variants with quite different fruits (if Callery pear ever does this), or completely different kinds of pear that are growing among Callery pears.