From HowStuffWorks.com:
Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x10<sup style="vertical-align: text-top; ">6</sup> joules (147,000 BTU), while 1 gallon of gasoline contains 132x10<sup style="vertical-align: text-top; ">6</sup> joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.
From another site:
A high expansion ratio is also one of the two key reasons for the efficiency of Diesel engines, along with the elimination of pumping losses due to throttling of the intake air flow.
I agree compression ratio doesn't (actually, can't) change with throttle position. It's a mechanical value. However, the text on limping losses would support goodvibe's statement about not being 'throttled' per se. Right intent, sideways explanation.