Complex sentence: What sets stuffing apart is that its purpose is solely to achieve an extreme state of fullness, unlike in feederism, in which the express intention of feeding a willing (or sometimes unwilling) subject over a period of time is gaining weight and lots of it.
Split into simpler sentences: What sets stuffing apart is that its purpose is solely to achieve an extreme state of fullness, unlike in feederism where weight gain is the primary goal. Feederism is a long term fetish primary associated with feeding a willing (or sometimes unwilling) subject over a period of time with the express intention of gaining weight and lots of it.