Why Imperial Fists are so small?
They were already large and well-established Legion during the Unification Wars.
They were running an sector-size interstellar empire, aka Inwit Cluster. Also they Terra and Solar Domain.
They possessed one of the largest and most powerful fleet in the galaxy entire.
They were one of the most enthusiastic Crusaders--it is said to be only Ultramarines and Word Bearers zeal was anything comparable. Moreover, they routinely mass-drafted from conquered population and garrison worlds. Sometimes, they impressed all of the promising youth and in many cases, they comprised entire companies or households of the Legion.
Their gene-seed is pure and stable, if not the most high quality.
Most of all, unlike their mortal rivals and foes(in other words, Iron Warriors), they are cautious at sacrifice and put their utmost efforts to minimize casualties.
Still, their size never exceeded 100,000 in whole duration of Crusade and at the eve of the Heresy, there are only around 98,500 Imperial Fists are presented.
If Alphas are stupendously big for the youngest latecomer, Fists are just mind bogglingly small for their history, force, role and indeed, repute and Emperor's favor showered on them.
Last edited by Santor; 10-31-15 at 10:23 PM.