User blog comment:B214/Volume 23 discussion blog/@comment-30924344-20170319035400/@comment-10330822-20170319060656

I think its the opposite, you aren't taking it literally enough. When Dragons want to fight seriously, they transform, why do you think that is? Its probably because those large bodies allow better usage of higher amounts of energy.

In short, Dragons decrease their size to conserve energy and move around easier, in addition to being able to blend in easier, but they transform to use their power. Issei is sort of the opposite, his body naturally transforms because of the immense amount of power running through it, and Akeno and Rias are needed suck out the energy to return him to "normal". In other words, to Issei his human form is "normal", while for Dragons, their dragon forms are "normal".

Eventually Issei may gain enough control over his demonic energy to transform and back at will. With that, he can harness even great amounts of power much easier.

This would kind of explain why he could just transform into a dragon and completely wreck a God-level UxL. The guy's dragon form is God-tier, like DxD level, except he doesn't even have to use his Boosted Gear, he just wrecks shop and leaves.

Now the real question is. Can Issei mix his Dragonification with his Boosted Gear forms? That would be interesting. In fact, that might be his ExE form, because with the powers of Great Red and Ophis, that's the only way I can see him getting stronger.