I have several secrets for growing large pumpkins. The basic essentials are good, like water, cultivated soil, manure or other nutrients in the dirt. Luck has a bit to do with it, no late freezes, no unkind weather. I don't let more than one pumpkin grow on each vine. Most people know this kind of stuff, but my super secret is birth control pills. I plant lots of birth control pills around the root of the pumpkin.
Well, I had this beautiful plant that looked like it was going to be a gorgeous pumpkin plant, so I gave it lots of pills early in the season, before the "pumpkin" was more than the size of a baseball, and I waited. You do a lot of that when you garden.
Eventually, it became apparent that the "pumpkin" was actually a squash, but it was growing so fast that I just let it go. It was a Hubbard. It grew until it was at least three feet in diameter.
Then it disappeared from my garden. I have no idea where it went, but I kinda giggle when I think about it.