I casually browse this subreddit without serious involvement, so I'll kinda give you a somewhat textbook standard idea as best I can.
I've fucked a skinny girl, fit girl, and a fat girl before, all of which very different experiences, obviously I'll be focusing on the fat girl but I'll make some comparisons to the other two.
Basically, her weight was less the issue, more the fact that it threw itself around constantly. One thing that was definitely obvious, however, was it was very hot. I don't mean hot as in sexy, like "Oh yeah that's so hot", no I mean LITERALLY hot inside there. It felt like my dick was wrapped in a wet, rubber heat blanket, no joke. But anyways, when she was on top, it felt like I was very constricted. My movements felt as though they did nothing, regardless of how much I felt it sliding, it was honestly less pleasurable than one would assume.
Her ass was another problem, considering that it's flat. She likes to think that her ass is the best thing I've seen. Truthfully, one of the worst asses I've seen in my entire life. It's very wide, and that's all I can say about it. Wide. Not at all pronounced, jiggles in all the wrong ways, just fucking awful. To compare the three asses of when they were ontop, I'll use sports balls. The skinny girl felt more like softballs, the somewhat bigger-than-baseball balls. It was small enough for me to grab with both hands fairly tightly, but it still had something there, and it was more than expected. The fit girl was more like footballs in the sense of a very easy grip, having a place for all my fingers to rest easily regardless of how much I grabbed it. Truthfully, she had the best ass I'd ever seen. The fatass, however, was more like flat basketballs because jesus fucking christ I couldn't grip it whatsoever. It felt like my hands were slipping off every single moment I tried, so I gave the fuck up and let her do her thing.
Problem #3, the smell/taste. It was not the worst, that goes right to the skinny girl for completely different reasons (bacterial vaginosis...uuuugh. Fuck). However, the girl in question is my coworker, of whom would generally like to fuck after work when she's all sweaty down there. Ugh. It wasn't HORRENDOUS, but it wasn't pleasurable either. I mean I could tolerate it to say the least, but then again I could tolerate my face being deep in sweaty pits, so that doesn't say much. Just awful. She basically can't even wash down there as far as I'm concerned, at least in the shower. She takes baths instead of showers because of this. Any time we'd fuck, the smell would not get out of my room unless I burned an incense or two for awhile and made sure the smell didn't get out.
Let's see, there's also the fact that no matter WHAT position we did, all I would hear is a giant, loud slapping noise. To compare: With the skinny girl, there was no slapping (only because it didn't fit all the way in since she's pretty small), the fit girl had SOME slappage but not a whole lot, at the time it was on my part since I was about 260 pounds at the time (at age 15). Take a slab of raw steak, put a condom on it and slap it against a water bed and that's about the sound you'd get.
To add a bit, her being bent over didn't help because as I said before, no fucking grip whatsoever. It was just awful. If I didn't have to pick the bottom of the barrel then I wouldn't have done it honestly, but meh, I'm an ugly fuck so what am I to do, I suppose. OH GOD THE NIPPLES, I DID NOT MENTION THESE. UGH. FUCKING FUCK. WHEN THEY ARE HARD, THEY ARE ABOUT AS BIG AS THE END OF MY INDEX FINGER AND IT DISTURBS ME TO NO FUCKING END. UGH.
Final thoughts: Just kinda meh. Not bad per se, but it could've been a lot better. I had to do quite literally all the work, she couldn't contribute whatsoever, so I had to move not only MY body but HER body. I'd take the skinny girl with vaginosis any day over her.
TL;DR: Couldn't grip her ass, giant nipples, uncomfortable, limited mobility, disgusting smell, too much sweat