All GSX Passengers are correct to the centimeter, the tallest guys is about 2mt, but most of them are way shorter so no, they are 100% accurate.
There are two different issues here you shouldn't confuse:
Passengers heads/feet stick out of the jetway:
This might not be an indication of the passengers being "too big", it might just be the jetway FLOOR HEIGHT hasn't been set correctly in the airport profile.
This of course discussed in the GSX manual, which clearly explain why it's not possible for GSX to know the exact height of the jetway tunnels, so it must be specified in the airport .INI file. The chapter in the manual which explain this in detail is at Page 71, and it's named "Custom Jetway Floor heights"
Jetway too small:
Assuming you (or whoever made the GSX airport profile, assuming you are using one) has set the correct floor height for all jetways on a 3rd party scenery, so you see passenger walking precisely on the jetway tunnel floor, but you STILL see their head sticking out, the only possible explanation is...the jetway has been modeled wrong.
And yes, there are some jetways from some 3rd party airports I won't name here, which we MEASURED (because we always test things in the proper way, before saying something), with the smallest tunnel to be just 1.5 mt high, which of course is absurd, since I never heard of a real jetway that forces you to crouch to walk it!
Obviously, OUR Jetways are correctly modeled with their precise dimensions, and all our passengers, even the tallest ones, have plenty of top clearance, clearly proving the passengers are fine.