If you are doing the exercises correctly-then your abdominal muscles are not getting "bigger". They may get firmer feeling-but they will not gain mass in that area. I agree that you may be losing inches in other areas such as your hips and chest-and it may make your tummy "look" bigger-but that is not the case. We don't lose weight evenly all over.
For instance-when I was at my highest weight, and I lost 40+ pounds before I got pregnant-I lost about 1 inch in my chest...3-4 in my hips, and 7 in my waist. My butt and boobs didn't get "bigger", but because my waist got so much smaller-it really made the hourglass shape stand out a ton more-and made my hips and bust more apparent. Fortunately, I liked it.
Measure, measure, MEASURE.
