I get that too once in a while. I will lose some weight and then my belly seems to get all mushy...then it gets back to normal...which is still soft mind you but not jiggly. Seems it just lags behind once in a while lol
I know it's getting smaller because I started at a size 22 at my highest and can now fit in a regular size 16 (a little bit snug still but it fits!) and some days, I find my tummy looks bigger than before.
But! I think it is precisely because the sides are getting more toned and smaller so the front just "looks bigger" in comparison.
One thing I have noticed too is that my boobs have won over my belly!
They're smaller too but look bigger than before because they stick out more now since I've lost weight under my arms and on my rib cage. lol Go boobs!
I can't wait for the day I can see those muscles tho! I know they're there! I can feel them! Just need to wait for that pesky belly fat to melt