But it's not their image, it's their essence. Imagine you summoned the essence of Hydaelyn. I doubt her -egi would be a giant crystal. It would be something that was of her essence. Something holy most likely. Titan is earth aligned, so his -egi portrays the essence of earth. Same for Ifrit and Garuda. You may not like how they were portrayed, but they probably shouldn't be portrayed strictly as smaller primals either way. It would just look odd, and would conflict with what they are.