Tinstaaflhas it right.
The easy definition of maneuvering speed is that speed where a stall will be experienced prior to reaching design limit load factor. In other words, you will reach aerodynamic limits before you overstress the aircraft, since you can recover a stall more easily than you can a bent wing.
The real structural load points of the aircraft, like the engine mounts and the wing roots (assuming primarily wing fuel tanks), see a constant mass (the engine for example), so LIGHT gross weight is more critical, since the wing at stall will produce the same lift regardless of mass, but at low mass the aircraft will achieve higher load factor.
Va varies with mass, and is slower at low mass as a result. In fact, it plots nicely along a V squared line, where the mass varies inversely with Va squared.