The source of these quotes come, not necessarily from the specific observation of corruption in the priesthood and episcopate, but the severity by which priests and bishops are judged at their deaths due to the ontological change of their souls, the character of the sacrament of Holy Orders, and the gravity of their duty to lead others toward Christ which is found in that sacramental character.
The mystic, St. Catherine Adorno of Genoa, wrote extensively upon the higher standard by which clergy and religious are judged simply through the very nature of their promises and vows. Even outside of this ontological change, the knowledge of God which is gained and experienced through the process of religious and priestly formation lends itself to much less leniency when judging sins as mortal. One of the criteria for mortal sins is full knowledge of the immoral action. It is much harder for the sins of priests and bishops to be considered venial simply because they didn’t know the reality of their actions.
Because of this higher standard of judgment alone, I would suspect that the percentage of priests in hell is larger than the general percentage of the laity in hell.
God Bless,
Br. Ben, CRM