A few random thoughts...
Everone has weaknesses, even priests.
Some things like smoking are addicting. Once you've begun, it's even more difficult to stop.
Although we know clearly today that some things are bad for you, that doesn't mean that someone in the past knew that information just as well. (Example, I won't touch artificial sweetners with a 10-ft pole, but many people think there is nothing at all wrong with them. Only time will tell who is right.)
Also, people's metabolisms change over time. A "fat" priest might have been in quite good shape as a young person--never needing to watch his diet or exercise--but then over time gets bigger and bigger simply due to a metabolism slow-down. It can really catch you off-guard and you might be clueless as to how to prevent or reverse the excess weight.
Point being, everyone is different and you should be inclined to always assume the best of intentions in everyone.