And no, I don’t mean “because he ate too many cookies on Christmas eve” or “he never gets exercise.” 
I mean, why is Santa Claus traditionally porrtrayed as being fat?
For the same reason the Ghost of Christmas Future is, it denotes prosperity.
The Ghost of Christmas* Present.*
Because tradition says that fat people are jolly. Ho Ho Ho.
Thank Coke.
ETA: linky
And, thank Clement Moore.
Fat used to mean healthy, strong and happy people.
It wasn’t that long ago when concerned parents nagged their kids to eat, eat, eat!
Father Christmas and St. Nicholas are traditionally pictured as being tall and thin.
Hell, the man only works one night a year…
to combat fat shaming and thin privilege
why do you have a problem with santa being fat?
The ancient peoples knew gaining weight gives a person a survival edge during periods of famine. Thats one reason people tried to gain extra weight when food was plentiful.
I recall seeing this happen on the 2nd season of Survivor. They cast these super fit, low body fat people. Several of them were runners. Within three or four weeks they all looked sick and under weight. By the end of the season the finalists were all emaciated. The winner Tina Wesson was almost skeletal. Imagine if these people had to live through a multi-year famine. They would have been dead by the first year.
Being thin and fit is only smart if the food supply is consistent year after year.
Clement C. Moore, Coca-Cola AND Thomas Nast
Santa is fat because he’s jolly.
And vice versa.
Marge Simpson: Con: you’re endangering your health.
Homer Simpson: Pro: I’m drought and famine resistant.
Exactly, when food was expensive and scarce, being fat was a sign of prosperity and success, essentially a prestige item. So a fat St. Nick was a big cheese (so to speak), someone important and respected. Poor people were thin people, because they could not afford enough food to get fat.
Just as, until Coco Chanel “invented” the leisure tan, it was a prestige and class indicator in Europe to be fishbelly white rather than farmer-tan brown.
Maybe if he took the stairs more often, instead of the chimney.
Because he only comes once a year.
Alternate joke: Why is Santa so frustrated? Because he only comes down the chimney. 
Because fat people are sexy!
Insulation - it’s REALLY cold at the North Pole.
I don’t think he needs a reason but since you are asking, there is nothing but fattening seal meat and candy canes to eat at the pole.