I was talking to my SO about that last week. It was like suddenly they didn't seem as heavy (or even as painful) as they had been. It happened with my first too and everything was fine.
I can see that but the feel of them they are not heavy or as full just really strange. My belly looked smaller today which was weird also. I can feel the baby move though.
Nothing to be concerned about. Your belly is getting bigger which makes them look smaller, and it could also be adding some support to them, which could make the fullness less obvious.