The research of more than 7000 people also found Christians have larger waists and are nearly one body mass index unit above non-believers. Male Sikhs were the second largest in the study.
Dr Deborah Lycett, who led the study, said the findings could suggest Christians feel it's okay to overeat because they do not drink excessively or smoke.
Gaynor Burton, the founder and director Christian fitness organisation Fit Fish, told Premier: "We have lots of socials at churches which often feature beige food... Pringles, quiches, sausage rolls. A lot of church meetings are sat down.
"Food attracts people into church. We often use that as a form of evangelism maybe.
"And perhaps there's an attitude that it doesn't really matter and that God loves us as we are so why do we need to focus on looks.
"When was the last time you heard a sermon on food and eating? We just don't think about it - it doesn't occur to us that we could be abusing our bodies in that way.
"But of course we wouldn't fill our minds with X-Rated movies or magazines from the top shelf.
"Churches should be encouraging movement and activity, and I think exercise classes are a brilliant idea.
"It's a great form of community and you get that buzz that's far greater than a sugar buzz from exercise."
