Jamieson, Fausset and Brown on Mark 11:21

21. And Peter calling to remembrance saith unto him--satisfied that a miracle so very peculiar--a miracle, not of blessing, as all His other miracles, but of cursing--could not have been wrought but with some higher reference, and fully expecting to hear something weighty on the subject.

Master, behold, the fig tree which thou cursedst is withered away--so connecting the two things as to show that he traced the death of the tree entirely to the curse of his Lord. Matthew (Mt 21:20) gives this simply as a general exclamation of surprise by the disciples "how soon" the blight had taken effect.