One of the common misconceptions is people think Cardio will burn muscle; you dont burn Muscle , you burn Fat! Keep the Cardio going, it will not affect you muscle mass just your Body Fat.
If you want to get big, lift heavy and Eat!
If you dont eat your body does not have the Fuel to recoup, rebuild Muscle fibers; If you do not train heavy, you will not break down/shock you muscle fibers into thinking hey we need to get bigger to cope.
Another very important factor is overtraining, over train and you might as well not train at all. Give your body time to rebuild and re-couperate, you can only break donw the Muscle fibers so far, you need to allow them build back up stronger , then you break them down again.
Unless you are taking a performance enhancement drug like Steriods, your body will not be able to re-coup between sessions; max 3 times a week for a natural bodybuilder. By all means go to the Gym and do some Cardio on other days but listen to your body.
Too many times I see people starting out performing advanced exercises, Remember compound exercises , Squats, Bench press, Dead lift, shoulder press are the key core exercises for mass.
Doing the Exercise with the correct form is more benefical than loading on the weight, dont worry increase in weight will come in time but focus on Technique, breathing and doing it correctly!!!