Actually, it wouldn't have to be heavier. It would just have to improve efficiency. The vast majority of the weight of the current amps is taken up by output transformers and heatsinks.

There are companies out there that are outputting 400wpc into 4ohms in substantially lighter and smaller form factors, but they're doing it using more efficient PWM digital amps. As these amp designs are perfected and gain more market acceptance, they will take over the class AB amplifier space.

Cheers,