Fat experiences

when I experienced being "the bigger one" (my 215 lbs Vs. her 90 lbs) all positions were comfortable also when I experienced being "the smaller one", all positions were pretty much comfortable
However when experiencing a really big weight difference (my 170 lbs Vs. her 396 lbs) I found it difficult to maneuver during, meaning it was really hard changing to a different position once she was already in one position...

convenient? doggy but prefer her on top (Me 200lb, her 475)

For us it's purely missionary because she's so fat she's immobile and helpless during sex.

She used to love getting on top when she was "just chubby", and occasionally she enjoys doing it to smother and crush me, but she gets out of breath so fast.

She used to be ashamed and embarrassed about being "too fat" to have sex like she used to. Until I let her know that I didn't care if I could never penetrate her again, because I'd rather have her bigger.

Then I started teasing her about getting too fat for sex...

She's recently started teasing ME about how she plans on getting too fat for sex!

depending on the size of my partner. body and genitals i prefer doggy or from the side

Traditional missionary is best for the wife and I :-) My equipment is surely long enough for doggie style, but her butt is too big even for it to comfortably reach all the way around it :-)