I don't think it's just the capacity but the power efficiency.
G700s has a really low run time, something like a week per charge as a heavy user.
The MX 2S can go about 2 months between chargers and the 3 should have an even better run time.
Hasn't been an issue for me, the cord lives on my desk and I just plug it overnight every couple of days. If I forget, grab a charged AA and swap it out.