User blog comment:B214/Volume 21 info & spoilers/@comment-26285797-20160715154901/@comment-24.113.252.213-20160715163045

The respective DxD Modes (G and L) are essentially the combination of their respective Heavenly Dragon's power with the special powers Issei and Vali each have within them (Issei: Ophis' "Infinity" power // Vali: his "Lucifer" power).

Yes, with DxD G having Ophis' "Infinity" power, it's potential is far greater than DxD L in terms of raw power. However, the only reason Issei is still alive after using DxD G, is because his body is made from Great Red's flesh. But despite that, Issei's "Great Red" body is still inexperienced in handling the "Infinity" power. Issei still needs to train his body to handle it. Ophis nerfed DxD G Mode by putting a "limit/cap" on how much of her "Infinity" power Issei can use, which will increase as he trains.

A good example to compare this situation to is Izuku Midoriya from "My Hero Academia". Izuku gets Super Strength, but he can't controle his power so he keeps breaking his arms and legs each time he uses it. So he has to keep training so his body can become a stronger "vessel" for the power. Look him up.