In my experience, as a man who hoe'd himself around feabie for the better part of a decade, the difference between someone in the 400-450 range and someone close to 600 pounds or more is usually pretty stark. Unless they're particularly tall, its almost universal that they have severe mobility and breathing problems in a way that is impossible not to take into consideration, and the sheer mass of someone that big being around you is simply transfixing.
Through the grace of god and the horniness of a man in his early 20's, I somehow managed to get with 4 people over 600 over the course of like 6 years. All were vastly different experiences, but did have some commonalities: they were at least 10 years older than me (very few 600+ women in their 20's), nearly all of them required a significant amount of messaging and/or a literal character reference to meet up, and walking more than room to room was completely out of the question.
The first and second were both semi-retired models from a big old-school ssbbw site. The first was just visiting sone friends and family, and after chatting for a few months, she invited me to her hotel. She stood to give me a hug when I arrived, but clearly that was too hard and she had to lay down almost immediately. She was very short with a pretty face and a massive apple shaped body. I nearly died eating her out (last time I tried eating pussy that big tbh) and afterwards flipped her onto her stomach for doggy style. Her pussy was incredible feeling, and we had the best sex I had with someone that size, even if she was only barely 600. She also ate a couple entrees of Red Robin casually afterwards, which was pretty awesome.
The second came from her reccomendation, and she happened to live in an assisted living facility nearby. She was easily at least 700 at the time, and without question the largest woman ive ever been with. This experience was a lot more intense, as she was either close to or nearly immobile, and I literally had to schedule around staff helping her with daily needs. I can't even describe how crazy it was to walk into her room and see her taking up nearly the entirely of her king sized bed, it genuinely broke my brain trying to understand seeing a human body that size. I brought her a fuckton of food (pupusas, mcchickens, donuts, candy, etc) and she burned through tons of it like it was air. Her kitchen was also stocked full to the brim with food, so it was clear her eating was being fully enabled even without a feeder. She had nearly all of her fat in her legs and belly, a noticeably less pretty face and kinda fucked up teeth tbh, but the sex was mindbending. I nearly lost an arm reaching in to play with her pussy, and because penetrative sex was basically impossible, I just fucked her stomach rolls till I came. The post nut clarity really fucked me up with that one tho, and I probably fumbled a second encounter as a result.
The third time was a few years later whilst visiting my hometown. There was a woman I had been talking to for a few years who moved back there between the last time I had visited. This one was new as before we hooked up, she picked me up in her car and drove me around so we could get acquainted first. She literally had to put her seat all the way down so her ass could have enough room to sit, and her fat completely overflowed against me the entire ride while we chatted. She had on okay face and a huge ass, I think she was some type middle eastern but in a way that looks super white. When I met her later, it was seeing her splayed out on a whole couch got me almost immediately hard, she was easily the second biggest person I had been with at least 650 and barely mobile. She was shaped more like a mountain, with slightly smaller shoulders, and a colossal everything else. Her legs were much nicer than the last person, and we were able to make non doggy style sex work, which was also an incredible first. She was also the most mentally well adjusted of the bunch, and the only one with a real job. Didn't eat anything in front of me tho, which was a shame.
The last one was just a few years ago, but easily the most effort I had to make. We met up near me as she was visiting family in my area, but we only met to get acquainted, which went pretty well tbh. She had me drive her car, and we drove around and smoked and chatted. She was a little over 600, but nearly all of it was in her belly, some ass, no tits at all. Kinda ugly face, but her vibe seemed cool, and she was easily the most into extreme feederism of anybody I'd met with. And easily one of if not the biggest bellies I've ever seen. A few months later, I drive 8 hours just to meet her at her spot, and stay for the long weekend. It was easily the longest time I was with someone, and I spent most of the time smoking with her and cooking a shit load of food. Let me say, I have never seen anyone consume that much food in 48 hours in my entire life. It must have been north of 25000 calories, and she just never seemed to get completely full. The vibes were weird tho, and her belly was kinda off looking, and I might sound like a pussy but it made sex genuinely a little hard for me. I did eventually manage to have sex with her, but it wasnt very good and our chemistry broke by the end of the weekend, even with all the feeding.
In hindsight, if you even have a stray chance, its worth it. The experience as a whole may not be great, but it will serve as permanent wank bank material in your mind for eternity. Dating however... I mean idk man, I'm probably okay with just a regular fat girl.
Hooking up with an ussbbw is like taking acid: euphoric, trippy, occasionally really uncomfortable and freaky, and probably best to just try a few times in your 20's, doing it a lot more after will make you super fucking weird. That being said, if that life's for you, fucking full send that shit baby.