I happened to love jeans and I'm always trying to dress everyone around me in the right jean for their shape. My very close girlfriend sounds like she has your shape - wider in the middle and small in the hips and leg area. She tends to like straight leg or skinny leg jeans (she wears a size 16 or 18). I always tease her about her pants bagging in the but area. She found some Old Navy straight jeans that fit her well - just a tad tight in the waist, but fitted every where else. Another jean that I think works for everyone's body shap is Levi's. They have an inexpense brand that is carried in Walmart, Kmart, and Target called Levi Strauss and go for about $20 a pair. They are stretch jeans and come in straight or boot cut jeans. They tend to run a little big. I typically wear an 20W in jeans but can wear an 18W in their boot cut. My only complaint with them and most jeans for me is their not long enough. The regular women sizes come in a long but not the plus sizes. Ashley Stewarts jeans are good fits too, as well as inexpensive. I, unfortunately, can't wear too many of the Old Navy jeans, I have a belly, a hugh butt and really big legs - their jeans are usually too big in the waist, fit in the butt area and too tight in the thighs.
I would suggest you try on a couple of sizes and styles at store and go with the one that fits the best. Don't worry about the size, just go for the best fit and look. Good luck!