I always seem to get this truck (I know it's for smaller planes) but I like the next size up better, so effectively want to delete the ability of this smaller truck to appear.
That "small" truck, under real world specifications, is able to tow an airplane weighting up to 185.000 lbs . This chart here (not exactly the same models as the GSX ones, but similar), shows the smaller FB-150 model can tow UP to a B757!
http://www.kalmarmotor.com/products/aircraft-typesHowever, if you like to see the larger models more often, you can alter the "aircraftWeight" value in the condition command in the SIM.CFG for the Pushback_Small model, which is now set at 175000 lbs.
For example, setting it like this:
condition = %(aircraftWeight)g < 100000 or %(parkingRadius)g <= 18
Will have the smaller truck appear only with planes lighter than 100K lbs, or in parking spaces with a radius smaller than 18 mt. The parking radius condition is there to prevent the larger trucks to appear in smaller parkings, where they'll have trouble maneuvering.