RT,
You're right about the gasser using a greater proportion of fuel vs air but keep in mind when the throttle plates on a gasser are closed at idle it effectively lowers the displacement. At idle the total volume of gasses that can be drawn into the gassers cylinder is less so the difference in actual fuel consumption is not as great as the relative fuel/air ratios would imply. At idle the gasser uses less fuel than at higher speeds because less of the mixture is drawn into the cylinder. The mixture remains pretty much the same at high or low RPM. It is the vacuum created by the gasser's closed throttle plates that the engine "works" to overcome. This is clearly not the only cause of greater idling efficency in a diesel but I believe it is easily the biggest.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold