I don't think the issue is Jill fucking another guy. It's that Jack is OBSESSED with his penis size and his ego cant take being upstaged.
Jakknife was way smaller and couldnt perform so no issue, but Girthmaster had way more girth (which basically every girl says is more important) while having good length.
As a dude that has also filmed some bigdick-focused content myself, it's actually crazy to see how consistent Jack is with utilizing all the typical camera tricks for maximizing how big your dick looks. Dude is legit the most committed minmaxer in the game.
-he always uses that same blowjob angle/setup so the girls look smaller since they stay out of the foreground when they stroke it, and the lens exaggerates the effect.
-He always makes sure that the base of his dick is right at the bottom of the frame (keeping your pubic area out of the frame in this position greatly improves the illusion, it's one of the best angles but u dont see it much elsewhere because it focuses too much on the penis instead of the girl), u can literally watch him fairly consistently readjust the camera when he accidentally drifts too low. like this:
-the vast majority of girls that he brings on are super tiny since when he finally brings on taller girls he doesn't look as ridiculously huge (e.g 6'2 rocky emerson, or even just 5'8 valeria mars)
vs
-Then there's the way he grabs his dick by pushing his hand as far into the base of his dick as he can in order to get every last mm of dick showing.
-and the way he almost always films missionary with him coming in diagonally from the edge of the frame, fisheye lens distorts edges + foreshortening
etc etc..
I'm not saying he isn't big or that he shouldnt do those things (i do like the content afterall), it's just to show that the guy is clearly almost always focused on making sure his dick looks as big as possible, which is why the girthmaster situation affected him like that
from looking at some of the scenes with performers that have worked with other hung guys and doing hand/dick comparisons and such, my guess is 8x5.5 which is huge but not quite as insane as stuff like that dakota tyler picture makes him look