During the Heresy there was no way 100,000 Astartes per legion at all.
Thousand Sons had maybe 1,500 at most.
Emperors Children had roughly 5,000.
All the other Legions had around 25,000-80,000 Marines IIRC.
Word Bearers had 150,000.
Ultramarines had 200,000, but it is rumored they are that large due to the other two Legions with missing Primarchs absorbing into the ultras.
By my math....
1,636,500 SMs max during the Heresy.
This is not using lower numbers of 20,000 for some legions, just the higher 80,000.
Not that big of a jump for Loyalist Marines alone in 40K.