On the left side you see the jeans as they came fresh out of the box. The thigh was pretty loose on this pair. The ankle was sewn so tight I had a lot of trouble getting her tiny pointy barbie feet through the holes.
I turned the pants inside out and found the inseam. The pieces are sewn together at the edges to prevent fraying, so I picked apart the seam up to the knee area in order to create a more straight-leg look.
Then I slipped the pants back onto the doll, still inside out, to check the fit again. The ankles were much improved. I pinched the excess fabric in the inner thighs and marked where I wanted the seam to go. I then tried them on my LBD basic #12 to make sure the pants would still fit a doll with the opposite leg structure.
I started the new inseam at the same point where I stopped picking apart the seam for the lower part of the legs. As I got near the crotch area I angled back out to where the factory seam is to prevent bunching that would be added by the extra fabric.
Turned them right side out again, and voila! the more fitted pair on the right is born. I think my alterations make the pants look a little more like the stock image, shown below. (sorry it is so tiny, I couldn't figure out how to make it bigger.)