Well-first I would like to say that you should consider yourself LUCKY-you have a classic hourglass figure it seems-like Marilyn Monroe and other classic beauties.
Everyone has a natural body "shape" and it isn't something that you can really change. Some women are apples (the women with the small buns, thin muscular legs-but when they gain weight it tends to be in their upper arms, chest, belly and back) or the pear shapes (where they stay slimmer up top and bigger all through their lower bodies).
You have what I like to call an "exaggerated" hourglass, which is actually what I have, and my sister in law has-although we are both larger/heavier than you are.
The reason I call it an exaggerated hourglass, is because I sew a LOT, and most clothing manufacturers and sewing patterns grade clothing into sizes based on standard measurements. Most of the time they grade patterns with a 10" difference between the waist and hip. If the pants fit a 28" waist well, then they will fit a 38" hip well-and so forth.
You have a 27 inch waist, and a 39.5 hip-so you have a 12.5" difference instead. Could I ask, what is your chest measurement? (Across the fullest part of the chest with a bra on.) What I think may be happening, is that it isn't that your behind is BIG on your frame...it is that you have a very small waist for your frame. a 39.5 hip is NOT large at all. That is nothing to be discouraged about-embrace it!!!

The "baby tees" and things that are snug around the middle were actually made for your figure.
As far as your weight goes-and your diet and exercise, you have lost 4 pounds in 17 days. That is 2 1/2 weeks. A HEALTHY rate of weight loss is 1/2 to 2 pounds per week-and you are RIGHT on track with that. Weight loss isn't like clockwork-you may lose a pound or two on the scale one week, and then go a week or two with no loss, then then lose a little something the third or fourth week. You have to give your body time to lose the weight.
Also, you are currently at a pretty low weight already...so your body could be telling you that you are already at a perfect weight for your body, and it may not want to go lower. A goal weight of 120 pounds is hard for many to reach the way it is...and a goal weight of 115 or 110 is even harder, or even impossible, for a lot of people to get to or maintain in a healthful way.
My advice would be to not concentrate on the scale weight at this point-because at a low weight your body may not want to lose more. What you can do if you are not happy with your body, is to maybe try to burn more fat and gain more muscle to make your body look better/firmer. You can do that by doing both cardio (walking/aerobics/biking/dancing etc.) and strength (a workout with weights/Pilates, etc.) together. You might do strength workouts 3 days a week, and cardio on the days inbetween, for instance.
It will take time, though, and you may have to give up relying on the scale for results at this point, since you are already at a lower weight. If you do lose a couple more pounds, since you are at a low weight-it is going to take a while. You may only lose 1 pound or so a month at this point. I would not recommend going lower calorie than what you are already doing-you might try to change what you are eating a bit though, go through it and see if there is anything you can change to make your diet cleaner and healthier.