I don't think pumpkin gets is name from the "-kin" diminutive.

Etymologyonline says: 1640s, "gourd-like fruit, of a deep orange-yellow color when ripe, of a coarse decumbent vine native to North America," an alteration of pompone, pumpion "melon, pumpkin" (1540s), from French pompon, from Latin peponem (nominative pepo) "melon," from Greek pepon "melon."

So the "n" has been on the end of the word for a long time.