User blog comment:Crimson and Darkness/Shin DxD 4 discussion/@comment-40183495-20200221101007/@comment-78.190.131.76-20200221150758

The biggest factor in the growth of Vari's power is due to the enormous magic power he inherited from Lucifer lineage and his constant training in his own style.

The reason for the enormous growth in Issei's power is due to the fact that many supernatural powerful beings have gained the attention, sincerity and support.