User blog comment:B214/Volume 21 info & spoilers/@comment-99.118.13.217-20160610101418/@comment-24.113.252.213-20160613164419

You remember how Ophis has the ability to bring out the full potential of a Sacred Gear. It's mentioned in her Powers & Abilities on her page. But do we know for sure that this "potential unleashing" ability is limited to Sacred Gears?

Maybe what she did was only bless Issei and Vali with her "Potential Unleashing" power. So in each of their cases:

Issei -- Unleashes Issei's "Infinity" power within him. Issei's body was molded by Ophis' power, so it is dormant within him. So when she unleashes his potential, all that "Inifnity" is unleashed, and the only reason he can withstand it is because his body is made of Great Red's flesh, the most durable material in existance (speculation).

Vali -- Unleashes Vali's "Lucifer" power within him. Vali is half human, so his ability to use his lucifer power is limited. So perhaps Ophis unleashes his full potential as a Lucifer.

At best, Vali's potential is Super Devil Class, while Issei's potential is Dragon God Class. So by this logic:

DxD G > DxD L

However, Issei can't properly controle his power to begin with, so if he has full, unrestricted use of Ophis' "Infinity", he'll just keep wrecking his body and suffering from "Can't See/Touch Breasts" disorder. So Ophis puts a cap on how much "infinity" power Issei can use that will change with how much Issei can handle.

Therefore, Issei and Vali are approximately equal for now. But with time, and training, Issei will come out on top.