Belias wasn't just an oversized Egi, he was the Egi of a former Elder Primal...buddy had to piece that power together from the collective experiences of ten different Allagan-era SMNs since, A) Belias has not been recently Summoned and therefore cannot be defeated in order to confront the Egi form in the normal way and B) The soul stones only hold the memories of the individuals who held that particular stone so if it took ten SMNs to fully summon the Primal, then you need the memories of those ten to at least form the Egi.