This topic has been beaten to death.

As for a short answer on why things are the way they are: the game's lore explains it in detail, both in the Summoner's storyline and within the main quest.

A lot of players disagree with the way summoners work, but you cannot deny that with the lore of THIS game... it makes sense.

We simply aren't strong enough to control an Egi larger than the one we have without becoming corrupted. It is my hope that with a level cap increase will also come a more powerful (larger?) version of the Egis to reflect our growing strength. This will not happen for a while, though.