Why do pilots' masks have more oxygen?
The pilots also need to use oxygen in case of smoke, fumes, or fire in the aircraft.  Thus, they need enough oxygen to be able to get down and land in a fire scenario, which is much more than a decompression!

Fire procedures have the crew on 100% oxygen at positive pressure to prevent them from breathing in smoke and fumes, while in a decompression, their masks mix oxygen with cabin air, in much the same way the cabin masks do.  This is also why the cabin masks are not used in smoke/fumes scenarios.