Remember: Purgatory is simply the last stage of sanctification. Sanctification in this life involves pain.
"For the Lord disciplines him whom he loves, and chastises every son whom he receives. . . . [And] For the moment all discipline seems painful rather than pleasant" (Hebrews 12:6, 11)
yet no one says that suffering infringes on Christ's sufferings. In the same way, the suffering during the final sanctification in no way infringes on Christ's sufferings or implies they were insufficient.
Quite the contrary!
The fact is that the suffering we experience in sanctification in this life is something we receive because of Christ's sacrifice for us. His sufferings paid the price for us to be sanctified, and his sufferings paid the price for the whole of our sanctification, both the initial and final parts.
It is because of Christ's sacrifice that we receive the final sanctification in the first place! If he had not suffered, we would not be given the final sanctification or the glorification to which it leads, but would go straight to hell.
Therefore purgatory does not imply Christ's sufferings were insufficient; rather it is because of Christ's sufferings that we are given the final sanctification of purgatory in the first place!
Peace