1. Her official art is one interpretation, and part of the fun of fanart is getting to see new and different ways people see her (this is something the cast has said they actually love about fanart)! People draw Jester in different outfits, with different hairstyles, different shapes of horns, different heights, and different weights.
It’s no different than how some people draw Nott with huge sharp teeth and bulging eyes (Sam has said he LOVES seeing “ugly Nott” art) despite that not being in her art, or drawing Yasha as 6′5, or drawing Fjord as a skinny twink, or drawing Beau with huge biceps, or drawing Caduceus with longer or shorter or differently-parted hair.
2. “jester is a pretty slender figure that is mostly muscle. because she is super strong.”
MENG SUPING, world record holder for 188 kilos/414.5 lbs clean & jerk (enough to lift a piano over her head)
HOLLEY MANGOLD, lifted 110 kilos/242.5 lbs (snatch) and 145 kilo/319.7 lbs (clean & jerk) at the London 2012 Olympics
SHAIMAA KHALAF, lifted 153 kilos/337.3 lbs (clean & jerk) at the 2017 Weightlifting World Championships in Anaheim
Or how about MARIAM USMAN, with a clean & jerk personal record of 155 kilos/341.7lbs set at the 2014 Commonwealth Games in Glasgow?
Big women can lift more than thinner ones. That’s a biological fact that athletes, coaches, and scientists all know. It’s why the sport is divided by weight classes– it’s not fair for a 5′4 110-pound girl to lift against a 5′4 170-pound one, because she’ll get her ass handed to her by the most basic laws of the universe: physics.
Of course, there’s exceptions to every rule, and strength is a subjective thing from sport to sport (these women wouldn’t do well in gymnastics, for example, or surfing– two more sports that also require a great deal of strength). I used weightlifting here because ‘lifting/swinging/moving heavy things’ is the core purpose of the Strength ability score, whereas many other sports focus more on things dealt with by Dexterity and Constitution in D&D.
So, to cite “she’s strong” as a reason why Jester can’t be fat? My problem with that isn’t that it’s mean (although it is a bit of a dick thing to say), it’s that it’s straight-up incorrect.
TL;DR: Fanart doesn’t have to match up with ‘canon’– that’s sort of the point– and the strongest women in the world are plus-size anyways.