As the article describes, the enzyme in the title is a "uridine diphosphate–dependent glucosyltransferase" which "catalyzes the addition of branched glucosides

As the article describes, the enzyme in the title is a "uridine diphosphate–dependent glucosyltransferase" which "catalyzes the addition of branched glucosides to compounds in stevia". So the sweet compound is not the enzyme, but "diterpenoids called stevioside and rebaudioside A" which are small molecules. Well small-ish, as you can see: https://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:9271 https://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:9271 with a formula of C38H60O18 it is 4-5 times the size of a glucose molecule.