Oddly enough, my wife and I were shopping for used task chairs over the weekend. My wife's petite, and has a hard time finding a chair she can be comfortable in (she is not happy with her current chair, and just started working from home). My chair is just really old and clapped-out.
She knows she likes the Aeron A size. Apart from being sized to fit her, it requires less weight to move up/down back/forth, unlike other chairs that are adjustable, but are designed for at least ~150 lb sitting on them. The only other chair we found that she liked nearly as well was the Steelcase Leap, which was about $500 used.
My own body is close to the middle of the bell curve, but I've got to admit, none of the chairs I saw are as comfortable or as well put together as the Aeron. Even used, it's $500+, and I have a hard time spending that much on my ass. But I probably will.
for the OP: my old chair is an older model Steelcase. it's actually a fine chair. If you can find a decent used office supply store, they'll probably have an aisle of them. I saw them around here for US$50 or so.