In today's politically correct world, you wouldn't nickname someone "Fatty," but in the '20s and '30s, the nickname was just another colorful example of the descriptive nature that baseball has always maintained.
At 5'11'' and close to 240, he wasn't exceptionally large by today's standards. But at the time, he was quite the sight. Fothergill wasn't only large and out of shape, but he was proud of it. Fatty managed to run quite well for such a rotund fellow and fielded his position with surprising grace. He was such a consistent hitter that he actually replaced Ty Cobb in the Tiger outfield and hit .367 in 1926.
In the book by Richard Bak, Cobb Would Have Caught It: The Golden Age of Baseball in Detroit, Charlie Gehringer recounts a particularly funny story about Fatty Fothergill.
He had a time keeping his weight in shape, but he still ran pretty good. In fact, I remember we were in Philadelphia once and we were getting beat about 13–0 going into the last inning when he hit a home run. He's rounding the bases nice and easy--and then when he gets to third base he comes running like a freight train and does a complete flip in the air and lands on home plate! Never saw him do that before.
Fothergill lived life to the very fullest and died of a stroke at the young age of 40. But one thing is certain: Those who had the chance to play with "Fatty" all genuinely loved the guy.