Not all "head popping" issues are really caused by jetways too small, in some cases it might be enough to the jetway roof height correctly in the GSX .INI profile, this usually fix most of these issues.
However, GSX passengers are surely modeled with realistic sizes, with the two tallest male adults being about 2 meters, the rest averaging 1.75 and most women ranging from 1.60 to 1.75.
Have a look at this video, the guy seems to be reasonably tall, yet he has lots of clearance, even in the last section of the jetway before the airplane door, and he only ducks a little bit when passing under the door.
We saw some 3rd party jetways that are clearly wrong, and not by a small amount ( we measured one case of a jetway section only 1.5 meters high, that's blatantly wrong ) so no, instead of asking GSX to feature unrealistically small people, it would be best if other developers modeled their jetways correctly.