My opinion is very similar to @avakann. In terms fantasy, there is no upper limit, and usually my tastes at that point launch right into the extreme.
In reality, I don’t really have a hard set weight in pounds or kilos, because different people gain in different ways. One person could comfortably be 342 pounds and reasonably mobile and capable, while another person could be 250 and feeling every ounce of the weight. Different sizes come into play as well; someone whose 5’11" would be able to get larger without major issues (generally) then someone whose barely 5’2". My partner’s health and happiness is the key concern, and if she wants to get bigger, smaller or stabilize, the choice is entirely hers.
I do have quite a soft spot for lots of soft, jiggly rolls, and my general feeling is “the bigger, the better.” Minor mobility issues (slower general movement, tired over short distances) don’t bother me, and are more often sources of enjoyment. Outgrowing things (clothing, doors, cars), again, is a major “YES.”
That said, there is a lot of problems with getting too fat. Outside of the obvious medical and hygienic reasons that have been stated before, there’s a lot that you simply can’t do once your partner crosses that threshold. Walking along the beach, going dancing, shopping dates, going to the movies, little things that seem so small but you feel quite a bit once you can’t do them.
I would never want to try and recreate the extreme weights in fantasy into reality, and I suppose I do prefer to keep some element of the fantastical in my feedee fantasies. If there’s not that bit of distance, I find it can very quickly turn from a case of “Oh Yeah!” to “Oh… yeah…”