Charles Ellicott Commentary Numbers 18:27

"And your heave-offering shall be reckoned unto you, as though it were the grain of the threshing-floor, and as the fulness of the winepress." — Numbers 18:27 (ASV)

And as the fulness of the wine-press.— The word which is here rendered fulness is the same which occurs in Exodus 22:9, and is there rendered the first of thy ripe fruits. It is used to denote either the fully ripe grain, or the produce of the vine. The tithe which the Levites paid to the priests was regarded in the same light as if it had been the produce of their own labour.