Two things: the typical "knight's armor" of the period in discussion was a chain mail hauberk (thigh length shirt) and chausses (maille leggings). It would have been as unfitted as a sweatshirt, with a 3-4 inch height difference not readily noticeable. I am unsure if there are even any surviving intact Norman/Viking/Saxon hauberks.
Second, armor is often displayed on a rack that does not take into account the extra material and play at the joints. When you add up these, armor has a surprising amount of expandability.
Oops. Didn't realize this was a two year old thread.