The size isn't the issue- it's the manifestation.
Ionion Hetairoi is a composite Reality Marble formed from the shared dreams of Iskander and his army working as one to impose the reality held within their hearts upon the world. They wouldn't be able to be summoned at all if there weren't so many of them, because it's the number of hearts working in unison which enables Iskander to create such a world without being a magus himself. He can't just summon one soldier, he summons Ionion Hetairoi.
That's what I'm talking about here- clarification on the method of the summon and the actual capabilities of the troops, specifically, and their status as Heroic Spirits or not might be nice to have.