It depends if you are talking about % increase in fuel economy, or absolue fuel economy.

An 1800 pound Insight ( hybrid ), will easily average 60 MPG.
I'll bet an Insight car, without hybrid IMA would still get 50 MPG.
So the Insight's "hybridness" is only a gain of about 20%.

Now take a Ford Escape V6 vs. a Ford Escape Hybrid, at 3800 pounds.
A V6 Ford Escape will average 18 MPG.
A Ford Escape Hybrid will average 31 MPG.
The Escape "hybridness" is a gain of about 70%.

Denver was the first U.S. city to purchase and put into use a fleet of hybrid electric vehicles.
Starting as early as 1996, Denver put a small fleet of Hybrid buses in lower downtown.
These buses never exceed 30 miles per hour and have to stop every city block, and navigate congested traffic.
The older buses, carried 75 people with standing room taken, and got 1MPG from diesel fuel.
The new hybrid buses are articulated in the middle, double long, and carry 125 people when standing room is taken.
The Hybrid buses get 4 MPG.
For a city bus, the "hybridness" is a gain of 400%.

Sounds to me like the bigger the better, and the Insight is the clear loser.

-John