Why does God punish a believer for a small sin, yet grant a transgressor respite for a grave sin?

What befalls a believer of calamities, pains, and sorrows is an expiation for his sins and a mercy from Allah upon him, as his punishment has been hastened in this world, unlike one whose punishment has been delayed until the Hereafter. The Prophet, peace and blessings be upon him, said: "If Allah intends good for His servant, He hastens his punishment in this world, and if Allah intends ill for His servant, He withholds from him (the punishment for) his sin until he meets with it on the Day of Resurrection." Sins are expiated by illnesses, diseases, and calamities, and by them, ranks are elevated and good deeds are increased.