ERE was able to raise large armies quite long : until the XIIIth century, an army of 20,000 men was an important expense but quite doable. It's really in the XIVth century that Byzantine Empire had lost too much territories (and so men and resources, being more reliant on alliances) to have huge troubles raising 10,000 men for large campaigns (which, by the standards of the era, is still an impressive force)