I am of two minds about this fandom's preoccupation with holding up Kirk as an example of fat acceptance. On the one hand, it's nice that we have all this fanart and writing celebrating someone who doesn't entirely conform to the body image the media has saturated us with for so long, starting in the 80s with hypermasculinized figures like Rambo.
On the other hand.
He's… guys, he's not fat. William Shatner was in great physical condition for the entire show. Yes, at some points he had more weight than others, but he was never fat, and he really wasn't even chubby.
The only reason people think of him that way is because the media has conditioned us to think of strong fit men as having no body fat whatsoever. They do this by literally dehydrating their male actors for days before shooting, to the point that it can be physically dangerous. Intensive workout routines combine with diuretics (drugs) and enforced dehydration to bring actors to their brink on the precise day of shooting.
Hugh Jackson had to dehydrate for 36 hours to play Wolverine, and he was on the verge of blacking out during many of his fight scenes because humans can only go three days without water.
Chris Hemsworth had to dehydrate for days, losing about a gallon of water to play Thor and needed recovery time when he was done so he didn't have to be hospitalized. Imagine Chris Hemsworth not looking good enough for your film, so you need to force him to lose a gallon of water to shoot a few shirtless scenes.
Not all actors are so lucky. Sometimes they do wind up in the hospital from these dangerous practices, and everyone who absorbs these unhealthy body images suffers as well.
Captain Kirk has never been fat, nor has he been chubby. While it's good that people can think about liking a chubby lead, what he should be is a model of a healthy, achievable body image for men.