I talked a bit about it on this forum a couple of years back, but my other fetish is semi-realistic hyper-pregnancy. I would not call it a side fetish, since 1) I am attracted to both equally and completely separately and 2) both were discovered at such a young age, I do not know which came first. Combining the two (e.g. a morbidly obese husband and a massively pregnant wife) would be distracting and difficult to balance, story-wise and masturbation-wise.
Oddly enough, each fetish is mostly gender-exclusive. With m-preg, I am like, where are the titties to feed those kids once they are born? And with female weight gain, I am not sure why I am not really into it. I think it is because the fat distribution and shapes are different.
Speaking of shapes, the other point of intrigue from my dual fetishes is that while both have to do with expansion, they involve vastly different forms. Hyper-pregnancy involves nearly-spherical, mostly-taut, and temporary expansion with offspring (as well as milk and amniotic fluid). Male weight gain involves rounded, malleable, and slightly more permanent expansion with rolls of adipose. The links between them are the struggle to heft and the natural, yet also unnatural, situation the characters find themselves in.
To piggyback on some other fetishes already mentioned, I believe the hyper-pregnant should absolutely be pampered. In my ideal fantasies, the morbidly obese should be treated compassionately, but not pampered. Scenes of embarrassment are generally a turn-on for me, particularly with male weight gain. Those episodes break the built-up denial, and the acceptance is a prime opportunity for emotions and occasional romanticism. While the majority of the content I consume is furry, that is merely due to the fact that I have found more fat-fur and preg-fur artists that better cater to my interests than their plain fat/preg counterparts. [I partially blame the website interfaces (DA vs. FA).]
Thanks to @THE_DIALOUGE for starting this, well, dialogue! I am surprised that, to my knowledge, there has not been a thread about this yet.