After thinking about this, I'm pretty sure the egi design is intentional. Primals are fully fleshed beings only when they have enough prayer and crystals, right? So one summoner can only do so much, and will only produce a fragment of what a full primal could be. Theoretically we could see an egi from a small beast tribe with few crystls, while we see Alexander is way bigger and more complex because of the amount of crystals used for him. (Or really, just raw energy from the land.) So I think the egi is partly a representation of the limits of a single summoner.

I say this because carbuncles and fairies are very detailed pets, and I am unsure of how carbuncles are brought into being, but fairies are already existing creatures that SCH bond with and summon to them from another area/realm, so they aren't an extension of the SCH's power, unlike egis. Carbuncles and fairies though, on a design level, all have "flat" coloring with glow/sparkles added whereas the egis all look like they are made of crystal and have the elements orbiting them, so maybe that texture has something to do with it.

Tl;dr: it seems like egis were intentionally designed to be unimposing, and if they were, I doubt the dev team will compromise what they want for the game, especially aesthetically.