Have you thin your pears? Thinning will definitely help increasing the size and quality of the remaining fruit.
While you may have one pear tree, your Harrow Delight could have been cross pollinated by other pears in the neighborhood including flowering pear trees.
My Harrow Delight has born a lot of fruit the past few years, but the size of the fruit has remained consistently smallish, even when there was less fruit, although it did occur to me that I should have thinned it this year! There have been no other houses with fruit trees, ornamental or otherwise, within 1/2 mile of us until last year. But you have encouraged me to try planting another pear tree variety to see if that will help increase the fruit size, along with thinning, of course.
I have some pears that are much smaller than others and although I’m not sure what the cause is I think it is related to not being well pollinated. I cut some of these and check for healthy looking seed. In some fruit there will be one good seed or seed that looks underdeveloped. I’ve added several varieties in an attempt to get better cross pollination and I’m getting more large pears but some still remain small. These are just my opinions and I don’t know if I’m on the right track or not.