Well, here's a few thoughts...
I don't think 25m is a depth where you'd really expect to seriosly freefall, if your buoyancy is set the normal way. So doesn't sound too worrying...You just need to get a little bit deeper
That's a good pair of lungs you have for your size, but with big lungs you'd actually expect a bigger buoyancy change. And given that you dive with a 7mm suit, it should be pretty sharp (in short the bigger the buoyancy, the bigger also the relative change).
But there's no defying physics. Either dive deeper or add more lead and you will freefall. It really is as simple as that.
That said, I think 7kg is a huge amount of weight and 7mm suit in most cases would be a little thick for constant weight. 5 or even 3mm and 2-3 kg weight might a better option?
If you just like to try how it feels to freefall, then yes, exhale less or put on more weight...
