How big is the MW,
The little one will use less when charging, But not half as much
20 amps at 14v 280 watts, is the same load, no matter the engine size
Yes the bigger one has more engine and generator head to turn
But it is not double the gas when under load , charging
Idle time running, is not used for comparison
And 50% load of the big one is more than double 50% load of the small one
They will both use almost the same amount of gas when charging
The big one will use a little more, but not double, perhaps 25% more gas than the little one with the same amount of charging load
Btw, the little 1200w will power a 600-800 cooking watt MW
