Tubby Lumpkins is another word for fat. Typically the people with lots of jelly rolls. Calling someone tubby lumpkins is the same as calling somebody Chloe Kardashian because they are both fat!
When fat people repeatedly tug on the front of their shirt to make it seem loser as not to attract the attention to fat tub of gue they have hanging off their front side. This is a very common occurance with people who are in denial that they are fat and refuse to buy larger t-shirts therefor they settle for the tubby tug.
Ryan has put on some extra weightlately and he constantly does the tubby tug because his shirt doesnt fit him anymore.