Hi Marisol!
Congratulations on losing 85 pounds. How inspiring.
Unfortunately there is no magic answer to help you slim your thighs. We all have our own unique shapes that we were born with and each of our shapes will repond differently to exercises. The key to helping to reduce your thighs is finding the right combination of exercises that seem to stimulate the best results. But before you get into some of this experimenting, there are some things that you should know that generally benefit everyone. I'll fill you in on those and then after that you can fine tune this information to work within your particular genetics.
First, to receive the health benefits of exercise you should do atleast three weekly cardiovascular workouts at 20 minutes per session. You should also strength train all the major muscles in your body at least one time per week to keep your bones strong. Second, you should eat a healthy and balanced diet. Your calorie intake needs to support your energy output. Since I don't get specific with diets, if your interested your best bet would be to contact a registered dietician to find out what your best food choices and portions would be for your body specifically. Third, spot reducing, meaning attempting to work only a specific area which troubles us in order to lose weight in that area, does not work. Fourth, over exercising with the hopes that "more is better" only sets us up for injury, which in turn set us way back in our progress. Fifth, variety in your exercise program keeps your muscles responding positively. If you don't have enough exercise variety your progress will slow down dramatically.
Understanding the above will help take some of the guess work out of designing your successful exercise program. But now the hard part is finding out what combinations work best for you.
As for step and weight training making your legs bulky, this is absolutely not the case for everyone. I would offer advice in that if you feel that your legs are getting bulkier from step aerobics(after giving it a fair try), then only do it once or twice per week. Also, if you feel that weight training is making your legs fuller(after giving it a fair try), then stick to lighter(but not too light)weights. Ok, I hope this helps you. Again, congrats on your weight loss success!
PS, Have you considered sending us your success story. If your interested, the details are on our home page.