Guys, sorry for my 767-300 "medium-statement"..!
According to this link, the 767-300 is in fact categorized as "Heavy":
http://www.airlinecodes.co.uk/arctypes.asp It turns out, however, that the developer of our Captain Sim B 767-300 for some reason has put it into the wrong "medium" category in his configuration. So this is what obviously caused all the confusion!
Anyway, even though not realistic, my solution of shrinking the wingspan in the aircraft.cfg is of course still an option if you are tired of not being aloud to park at medium gates (max. radius 23 m.)