Here is the line in sim.cfg that I assume has to be changed.
There's nothing that must be changed. That line says the small pushback will appear on any airplane that weights less than 175000 lbs OR if the parking has a radius smaller than 18 meters.
Since a Learjet is surely lighter than that, and since there's an "OR" condition, meaning either of the conditions must be true in order for the vehicle to be selected, it's not possible it won't appear.
It's likely that other trucks with no conditions are competing with it (AS, TPX and TMX don't have conditions) so, you might add a new "condition" line on THEM, in order to exclude them for lighter planes.
For example:
condition = %(aircraftWeight)g > 50000
Added to all SIM.CFG of the other trucks with no conditions, will leave the small truck as the only usable one for planes lighter than 50000 lbs.