User blog comment:B214/Volume 23 discussion blog/@comment-172.58.17.194-20170329205153/@comment-82.60.161.59-20170329232239

if in 30 years Issei can rivals Great Red's strenght there would have not been any point about keeping the others in the barrier with the Trihexa, because then Issei Vali and Shiva could beat the Trihexa without any problems. Issei possess only a crumb of Great Red power. He possesses a bit of his flesh and a bit of Ophis power, the amount used to made his body, which is a easy thing to do for her. It's infusing them in his Sacred Gear that he becomes so strong, but it still nothing compared to Ophis and Great Red. Remember that the current Ophis's power is only twice of the Albion and Ddraig together and like this she feels "weak". When she was complete even the True Longinus piercing her body was less tedious than a fly on her head.

And his DxD G was nerfed because it wasn't his own power! Ophis combined her power to his! So that wasn't really Issei's strenght, but Issei borrowing Ophis power (not just the little fraction he had on his own body)