User blog comment:B214/Volume 21 info & spoilers/@comment-37.157.219.66-20160402232417/@comment-24.113.129.70-20160403195101

I thought Ophis' power of "infinity" worked something like this:

She (and now Issei in DxD mode) had an "Infinite power supply", but she(they) have a fixed "power output." And that "power output" is dependant on how much their bodies can withstand.

Maybe what Ophis did was lower the "power output" of DxD mode so that it doesn't put too much stress on Issei's body. Remember, he may have a body made of Great Red's flesh, but he hasn't trained it enough to  use DxD mode freely.