More specifically, (
via Lexicanum):
The first batch of Alpha Legionaries were notably tall and strong even for members of the Astartes, physical attributes which suited Alpharius' focus on misdirection. For the Primarch put into place a directive that, as far as possible, all Alpha Legion marines had to attempt to look alike; and the visage they patterned themselves on was that of Alpharius and Omegon. As a result, all Alpha Legionaries were at the very least shaven headed, with many going so far as to have cosmetic surgical alteration. Their height also made it easier for them to be confused with the Primarch, although the twins were still the tallest in the Legion. When asked by non-Legion members, all Legionaries gave their names as "Alpharius", even when more than one was present.