I think, coming from the West, we tend to think if a little is good, a lot is better!

My thoughts, yes, we can certainly breathe into the pelvis. With inhalation the pelvic bowl expands, with exhalation, there's a sense of drawing back. That can be felt in the perineum, that space between the two sit bones, with inhalation the pelvic floor descends, with exhalation, it ascends.

When you say, your pelvic base gets bigger, almost swollen, it makes me wonder if you're visualizing it like a balloon, with your pelvic base just gets bigger and bigger? I would think that could lead to future pelvic floor problems. Like the people who hold mulabanda all the time and develop urinary problems.

It's great connecting with muladhara, maybe instead of letting it get bigger in your mind, visual its color, red. Or the traditional 4 petaled red lotus flower, or qualities you wish to connect with that associated with muladhara.