IMO, the problem is that many of us want our trousers to fit like our jeans. To further complicate matters, manufacturers have added stretch to traditional trouser fabrics, making it easier to squeeze into smaller sizes. The result is wrinkles and whiskers on the thigh area for many women. I also think that, since fashion has dictated a tighter fit for the past decade or so, many women are just not used to wearing a looser-fitting pant and equate extra fabric around the thigh area with the garment being too big.
Since trousers are an adapted mens-wear look, I think they look better if they fit closely around the hips, but more loosely in the thigh area so the pant falls in a straight line to the floor. This fit feels very different than a jeans fit which usually cups in under the butt and fits tightly on the thigh. To avoid the "diaper butt" syndrome with this extra fabric, it is vital that the crotch seam in a trouser fits correctly; if it droops, you'll get that saggy look in the back. The cure is not to downsize, but to have the tailor raise the crotch seam. Adjusting the crotch seam and the waistband can also improve the fit in the front so that the fabric doesn't pull in under the tummy.
A well-fitting trouser can be a very slimming look, even on a pear-shape figure as long as the fabric can fall in a straight line from the upper hip. The reality, however, is that most pear-shaped women probably need to buy the larger size and have the pant adjusted by a good tailor.
Angie has some good tips in this post: http://youlookfab.com/2010/03/.....ess-pants/