Im 5 feet 7 and usually 8 stone 3 and a size 8 (im a little more after xmas!). I was 11 stone at 40 weeks pregnant and lost it all quickly. But 10lb of that was baby lol.
Weight is different on different people. This is what the dietitian told me both times i was referred in early pregnancy (due to low bmi). She had no issue with my diet or shape and said bmi is just a guide. At 9 stone im a size 12.
Ieat throughout the day (such as grapes, carrot sticks, an apple, thins like that) and so dont have huge portions at meal times.
I dont necessarily agree exercise is the beat way to lose weight really, as you get hungrier! Exercise is for health and toning up. If suggest really look at your diet (i never have take out. Makes me feel physically crap and lathargic after) and crucially portion sizes! A portion is the size of your fist (palm if it is fish).
Good luck! Summer bodies are made in winter as they say!
And remember it doesnt matter if you get to your goal as soon as you want, just that you get there.