If the snake in question was eating one rat a week, every week for the last owner........................... I would bet to say their husbandry was spot on. :gj:
What you think is "huge" might not be. Most people cannot tell a weaned rat from a small or a small from a medium until you weigh it out in front of them.
If it is eating let it feed, in time you will be asking why your ball wont eat. Its in their nature even if you don't think its right.
Photos would be nice to see so we can make a more educated guess.
Also keep in mind they are just like people, each eats and grows at their own pace.
also: I haven't seen anyone mention it yet but you want a rat that is about as wide around as the thickest part of your snake.