When apple trees are very vegetative, the growth quickly shades spur leaves and fruit while also growing root vigorously. At some point the growing shoots apparently stop serving the fruit so much as leaves, wood and roots. The shade this growth casts permanently kills the ability of shaded leaves to photosynthesize, and these are the leaves that can serve fruit (size and sugar content) and next years flowers best.
I believe summer pruning can control a lot of the negatives of vigorous rootstocks, but it requires more work than more dwarfing ones. I wish this opinion was based on solid research, but the industry doesn’t really care about how fruit tastes (brix) so much as appearance. Summer pruning is recommended primarily as an aid to getting better color and reducing summer fungus pressure. (I believe pruning in mid to late spring improves biennial baring.)
You can prune the top growth, but roots are a different matter, and excessive roots may contribute to more watery fruit in the humid region. This I’d like to know.
Meanwhile, I need to grow vigorous trees to keep ahead of the deer.