Board Thread:Warhammer 40k General Discussion/@comment-91.224.27.226-20131031140001/@comment-3258762-20140106142649

Hardly. Heads can be shrunk, skulls are made of bone, and bone doesn't shrink. Any process in which heads are shrunk involves removing the skull.

Further, the 2nd Primarch was found after Horus and Leman Russ and the 11th was found long after, and most Primarchs matured in about 3 years.