Hi there!
To tone muscles around thigh and the stomach regular low grade exercises that target the region are best recommended. Aside classic abs exercises, straight leg lifts and side leg lifts would also be beneficial.
Hope this helps.
Take care!
I used to compete as both a bodybuilder and also as a national level power lifter - there really is only one thing that brings out abs - strict diet and cardio. All the crunches and sit ups in the world will not help. I recommend following a high protein low carb diet for 16 weeks along with a 30 minute cardio routine (running is best) at least 4 times a week - also continue with weightlifting as the promotion of muscle tissue assists with keeping the metabolism strong. Best of Luck.
If your building muscle but not losing weight, your belly will remain the same size, even get bigger if you are doing quite a lot of weights with abs (only really extreme body building excersises that is). But usually the best thing for a flat belly is cardio as well, this helps burn the fat off those areas. I warn you though, if you lose all your fat you run the risk of damaging your bones and losing your period (sorry to be so blunt - seen this happen heaps). So if you do decide to lose weight by cardio, make sure you stay in the healthy fat percentage range.
However, I wouldn't worry about it too much. It also depends what body shape you have, some people store more fat on their belly. And it depends what you define as big, some people just have a warped perception that their belly is too big ;) So if your healthy, I'd keep up a good level of exercise and a balanced diet, but don't go extreme.