That's exactly what I was wondering. I suppose the change in his size from Tommy Boy to Black Sheep to Beverly Hills Ninja follows the trajectory of his drug/alcohol use. Thanks for the info, I really want to read those biographies.

Chris was at his healthiest during Tommy Boy. When on SNL, Lorne Michaels was very strict with him and was adamant that he remain sober for the show. He partied at night, but not to the degree that he did before and after he was on the show.
Based on the biographies I've read, Tommy Boy was a time when Chris was obsessively drinking coffee. He and David Spade would fly to Toronto to film and back to SNL to perform throughout the whole process. I imagine he was more sober during Tommy Boy than any other picture.
It was said that Chris would remain sober during movies and then binge on drugs and alcohol after the film was finished. Since Tommy Boy was filmed when he was under Lorne's observation, it makes sense that he was more in control of his demons and weighed less as a result. Early SNL films (like Airheads) were during a time when Lorne hadn't gotten to him about his drug-use and drinking yet, I believe.