The Summoner quest line explicitly states that the Egi are a manageable size. A larger Egi may still be manageable, but could become unruly. It wouldn't kill us. The mana it takes to perform an Egi Summoning is but a portion of our overall mana, or aether reserves. Yes, overusing our aether reserves would kill us, but more than an Egi killing us? Unlikely. Now a full Primal Summoning is impossible for a Summoner, mainly because while an individual can have enough worship (lookin' at you, Greg), an individual requires crystals because one's own aetheric reserves are not great enough to perform the Summoning alone. Could a level 100 Summoner do it with ilvl100billion and tonze upon tonze of piety materia melded in? Maybe? But to claim that for a single Summoner to summon something greater than an Egi would cause death, I think is too far.