I have been working out for a month and have already seen results which is great but I have a small dilemma - my legs are getting bigger! My ultimate goal is to get smaller - fit into my old jeans and maintain muscle definition. I have been doin 4 sets of 15 with light weight and 30 mins of cardio before all of my workouts. Is it my diet I need to work on??? What do I do?!
Yes, it's your diet, most likely. You need to remove the fat layer on top of those muscles you are working.
Have you been dieting? What does your daily food intake look like? If you've been maintaining your weight for a couple weeks, you are eating around maintenance, given the level of exercise you are currently doing. So now you need to increase your caloric deficit which can be done with a combination of eating less and increasing your cardio.
You mentioned you do 30 minutes of cardio before workouts - how many days/week do you do that. (Plus, I would never recommend doing cardio before weights. Ideally, you'd do it on separate days or after your weight workouts - not before).