I'm 5'9 and apple-shaped, so I have no butt or hips because I carry my extra weight in my gut. So my pants sizes are always smaller in comparison to my shirt sizes. Even though I'm a comfortable size 8 jeans right now, I still wear large shirts to hide my gut. A medium shirt would make my stomach bump obvious.
size 15 - 216lbs
size 13 - 190lbs
size 12 - 170lbs
size 10 - 165lbs
size 8 - 159lbs, fit into a tight size 6 right now.
size 6 - ???
I really have no idea what my ideal size should be anymore. Every time I think I've got it figured out, I drop another size.
The last 20lbs before you reach your ideal weight makes the pants-o-meter go crazy because you finally start rapidly losing inches in the gut or hips.