Sometimes we don't see the forest for the trees. This question seems to be a case in point. Just consider what the Eucharist actually is. The Eucharist is the greatest sacrament because it is the Lord, Himself!
We aren't able to measure grace, but God surely can. When we say that Mary is full of grace, we aren't so much measuring it as recognizing that she so very honored by God, Himself. Here's more on grace:
Fr. Vincent Serpa, O.P.