I think there are a lot of things that can make stuffing so arousing.
The first thing that comes to mind is the genetic or social wiring of a person in the sense that they associate eating and sex as the same thing or connected. If that is the case it is no wonder that stuffing is arousing. The fuller your belly gets with food, the more it feels like a climb to an orgasm, hence more erotic and aroused the person gets.
Second, I believe that it also has something to do with a subconscious feeling of being a bit helpless. If you're a sub (which not all gainers and stuffers are), but if you are, you just stuffed you belly full of food. It is all distended out in front of you making it hard for you to move and walk around, and the food coma may be starting to set in. If someone were to chase you, would you run away or even be able to run away? Additionally, if someone else is feeding you, not only are you enjoying great food, but the guilt is taken off of you and placed on the feeder per the dom/sub relationship. Also for a feeder/feedee relationship, it's arousing for the feeder to be doing something to the feedee that will physically change his or her appearance. The feeder's actions are extertion of control and he or she is reminded of that with every pound, curve, and roll on the feedee. Likewise, the stuffing for the feedee may be arousing because it is an admission or relinquishment of control they have over one of the most important things they have, themselves or their body at the least. And they are reminded with every pound gained and tummy stuffed.
Then there is stereotypical guy theory. Bigger is always better.
But I think that needs a little more expansion so I'll say, Bigger, rounder, and thicker are always better.
As for is sex better with a stuffed person, I'd be inclined to say that it has to do with their arousal to it. Additionally, if you think about it, they just stretched out their belly so it is very sensitive. Sensitive skin creates another erotic area and it's big one that may or may not be pressing down on other sensitive areas of the body. Plus it gives way to sexual belly play which is always fun and maybe even if your partner likes pain you can use their stuffed belly for that. The increase in libido may be a result of adrenaline and like I said earlier, the climb towards orgasm.
With all that said...who knows? I could be way off. LOL