Objection 1. It would seem that one miracle is not greater than another. For Augustine says (Epist. ad Volusian. cxxxvii): "In miraculous deeds, the whole measure of the deed is the power of the doer." But by the same power of God all miracles are done. Therefore one miracle is not greater than another.
Objection 2. Further, the power of God is infinite. But the infinite exceeds the finite beyond all proportion; and therefore no more reason exists to wonder at one effect thereof than at another. Therefore one miracle is not greater than another.
On the contrary, The Lord says, speaking of miraculous works (John 14:12): "The works that I do, he also shall do, and greater than these shall he do."
I answer that, Nothing is called a miracle by comparison with the Divine Power; because no action is of any account compared with the power of God, according to Isaiah 40:15: "Behold the Gentiles are as a drop from a bucket, and are counted as the smallest grain of a balance." But a thing is called a miracle by comparison with the power of nature which it surpasses. So the more the power of nature is surpassed, the greater the miracle. Now the power of nature is surpassed in three ways: firstly, in the substance of the deed, for instance, if two bodies occupy the same place, or if the sun goes backwards; or if a human body is glorified: such things nature is absolutely unable to do; and these hold the highest rank among miracles.
Secondly, a thing surpasses the power of nature, not in the deed, but in that wherein it is done; as the raising of the dead, and giving sight to the blind, and the like; for nature can give life, but not to the dead; and such hold the second rank in miracles.
Thirdly, a thing surpasses nature's power in the measure and order in which it is done; as when a man is cured of a fever suddenly, without treatment or the usual process of nature; or as when the air is suddenly condensed into rain, by Divine power without a natural cause, as occurred at the prayers of Samuel and Elias; and these hold the lowest place in miracles. Moreover, each of these kinds has various degrees, according to the different ways in which the power of nature is surpassed.
From this is clear how to reply to the objections, arguing as they do from the Divine power.
Source: Summa Theologica (New Advent)