As far as I remember GSD females are (when it comes to height at withers) from 55-60 , males are 60-65 not more. Vlcaks height at withers is:females 60 and up, males 65 and up. Of course there can be smaller vlcaks, it happenes.
Slimmer may be the wrong word, lighter I guess would be better. Vlcaks are lighter , their bodies are shorter, the angulation of their legs is different more of a wolf. Thus they are faster, they have better endurance, they regenerate more quickly. But as I said before, there can be heavier vlcaks, it very often comes with the height, not always, but very often
thus they are taller but also heavier, which changes their physical abilities.