As reported in the 1883 Daily Prophet article “Bring Back Our Baskets“, this change by the Department of Magical Games and Sports is made to ensure fairness and consistency (QA6). The change does make the game more fair but was not appreciated by some fans, who relished the fact that their team could get an advantage by altering the sizes of the baskets.
“Baskets, as you may have noticed, come in different sizes. We have found it impossible to standardise basket size so as to make goalposts throughout Britain equal. Surely you can see it’s a matter of fairness. I mean, there’s a team up near Barnton, they’ve got these minuscule little baskets attached to the opposing team’s posts, you couldn’t get a grape in them. And up their own end they’ve got these great wicker caves swinging around. It’s not on.”
— Ministry Representative explaining the change (QA6)
Perhaps Rowling was inspired by the early history of basketball, which takes its name from the the elevated fruit baskets which originally served as goals (Wikipedia). -BB