Lourdes:
Love your picture of you and baby, by the way. You look great. From your photo you seem to be a slim woman, for whom building a lot of muscle mass would be difficult, in other words, a hard gainer. Would you agree?
You know, you can gain strength without looking appreciably bigger. First of all, with the heavy weights, it is increased strength that you will gain. Muscle tissue is very dense and it is possible that while you seem to have lost muscle size to your own mind, perhaps it is that you have become leaner overall? Your muscles have gotten stronger, more dense and you may have lost some fat that was covering the muscle and that made it look bulkier?
If you are a hard gainer, it will take a while, even with heavy weights, before you can see gains in muscle mass. How long have you been using the heavy weights for?
If you cut down your cardio because you were getting too tired and it was too time consuming, then that was not a bad idea. But you seem to have enjoyed the results of your more varied lifting program, alternating heavy with moderate weight, so why not go back to it? It's fine for experts and books to state that reducing cardio and lifting heavy will increase muscle mass, but everybody's body is different. We may not, as individuals, respond as expected to prescribed fitness plans. Most of us experiment all the time, trying to find that ideal combination of weights and cardio that will produce the results we want. Sounds like you still have some adjusting to do.
I am a hard gainer, so I always work with heavy weights, but I prefer to work each muscle group twice per week. I enjoy greater results that way, plus I feel better and get much stronger. I tend to do S&H once per week, plus either PUB or my own weight training mish moshes which I have formed from blending GS/CTX for upper body and GS/PLB/LL for lower body, in which I do multiple and continuous drop sets so it's a combo of strength and endurance lifting.
As far a how much cardio I do, the deciding factor is not, "what's the correct ratio of cardio to weights to build muscle mass/lose fat?" or whatever. The deciding factor is always, for me, how much energy I have, how little sleep I had the day before, the vagaries of my kids' schedules and whether I can fit it in, how stressed out and badly in need of a sweat session am I, and basically, how bored or interested am I by the prospect?
What I mean to tell you is that you should not over-think it or stress about it. Work out to make yourself happy and healthy, do what makes you feel good and what you have the time and energy for: I tend to let the results take care of themselves.
Clare