All vehicles in GSX are selected randomly from a list of suitable candidates, with "suitable" meaning in case of the Pushback truck, if the airplane weight is not too high for that truck to tow it, according to real world specs, and in case of stairs, if the stair can physically reach the door height. So all the vehicles are correctly selected, according to their real life specs and measurements.
If you don't want to use some vehicles, you can edit their conditions in the SIM.CFG file, as explained here:
https://www.fsdreamteam.com/forum/index.php/topic,28098.msg183981.html#msg183981and here:
https://www.fsdreamteam.com/forum/index.php/topic,28098.msg183981.html#msg183981