Being big or heavily muscled can be a disadvantage in fighting sports like boxing and martial arts, but being strong and powerful is always an advantage. With contact sports, you want to be strong and powerful and agile enough to be able to use that muscle power on your opponent.
As it isn't possible to gain muscle size or strength while in an energy deficit and that it isn't possible to lose fat whilst in an energy surplus, the trick to weight loss whilst gaining muscle is to fluctuate your body between energy surplus and deficit at different times of the day, or on different days of the week, through diet and exercise. However, if your goal is to lose body fat as efficiently as possible, then it is not possible to gain muscle at the same time as losing body fat, as energy reserves are far too insufficient for muscle growth. Here the priority is maintaining muscle mass. So, as you're wanting to maintain or even gain some strength whilst losing fat you'll need to be in energy deficit for much of the day, but have excess calories for growth in the hours post weight training.
Meals must be small but regular, and in order to keep your strength, it is essential to keep protein intake high. The key to effective fat loss lies in careful manipulation of your intake of carbohydrate foods, i.e. carbs should be low, but not omitted. Consume low glycaemic starchy carbohydrate foods regularly, but in small portions only, and avoid treats and junk food. For more information see here. See our Glycaemic Index Tables for GI values of foods.
Following a regimen such as the meal plan below should keep body fat down, both weight and cardio training must remain intense. You will be able to continue to train hard as your calorie intake will not be mega low and you'll have regular carbohydrates. Sparring, pad work or cycling will help you achieve results; try to up the intensity and make sure your fighting training is done on a different day or at a different time of day to weight training so as not to interfere with nutrients required for strength gain.
As with all the meal plans this is merely a guide and must not be stuck to rigidly! You must eat a variety of different meats / fish / alternatives, complex carbohydrates, fruit and vegetables every day, and drink plenty of water. Adapt the plan to suit your own needs according to your results in order to attain a steady loss of body fat and respond to how you feel strength-wise. Varying portions from day to day, along with hard training will help maximise progress.