Why are inverter generators so much more compact than regular generators?

And this is because, with the throttle plate more open, there is less pumping loss drawing the air into the engine.

Really? I thought an engine gets its best fuel efficency at part throttle when its pulling a high vacuum?

you’re using the word “efficiency” when you mean the word “economy.” car engines get their best fuel economy in the situation you describe because they’re running steady state and likely not putting out more than 30-50 horsepower. but their actual thermodynamic efficiency (the amount of output power per unit of fuel consumed) is horrible because they expend a ton of energy just trying to suck air through a barely cracked open throttle plate. try breathing through one of those little coffee-stirrer straws and see how much more effort you have to put into it.

a gas engine’s peak thermodynamic efficiency occurs when these conditions are met:

  1. the engine is at wide-open-throttle (WOT)
  2. the engine is operating at the RPM where it produces peak torque

diesels don’t have throttles, so one part of their efficiency advantage over gas engines is due to that.