imho, sex with slim girls is prolly due to manueverability. also, slim girls tend to have a easy job of maintaining a Waist-To-Hip ratio than plus sized girls.
but then again, plus sized girls have a better chance of having a higher class cup size too.
im lucky enuf to have a girlfren who is plus sized and has a nice WTH ratio. im happy cos when i hug her, i feel warm. as for the manueverability, i just use that as a push factor for me to break my own limits in the gym
i cant say that for a skinny girl though. and thats just my 2 cents.