The pushback truck kind is chosen according to aircraft weight AND parking radius size.
As indicated in the sim.cfg of the small Pushback truck, that vehicle is configured to appear if the total aircraft weight is less than 180.000 lbs. Since the MTOW for a 737-800 is 174.000 lbs, GSX is correct assigning the small truck with a fully-loaded 737-800.
AND, there's an additional constraint on it, that it will be chosen if the parking radius is smaller than 18 meters, because the larger truck will have trouble maneuvering in the smallest parking spots.
The larger trucks appears if the airplane weight is more than 170.000 lbs AND the parking radius is larger than 18 meters.
This means, if your airplane weight is less than 170.000 lbs or the parking radius is smaller than 18 mt., you will ALWAYS get the small truck.
Between 170.000 and 180.000 lbs. you will get an equal chance of a small and large truck IF your parking spot is larger than 18 meters.
If the airplane weights more than 180.000 lbs AND your parking radius is larger than 18 meters, you will always get larger trucks.