Dummelow writes: " The vegetable or herb, not the so-called mustard tree, is meant. In hot countries it sometimes grows to a great size. The Jerusalem Talmud says, 'There was a stalk of mustard in Sichin from which sprang out three boughs, of which one was broken off, and covered the tent of a potter, and produced three cabs (12 pints) of mustard.' Rabbi Simeon said, 'A stalk of mustard was in my field, into which I was wont to climb as men are wont to climb into a fig-tree.' Although the mustard seed is not really the smallest of all seeds, it was so in popular estimation." (Dumm. pg. 673)
Mark's version says that the mustard tree is "greater THAN all herbs" and Matt's version is the same as Webmaster writes, "greatest AMONG herbs". "Greater than" is different than, "greatest AMONG." To make my point, I am larger than an apple, but that doesn't mean that I am the largest among apples! NEB and J.B. Phillips write of the mustard tree, "---it springs up (grows up) and grows taller (becomes bigger) that any other plant." RSV, "becomes the greatest of all shrubs", NI and NAS, "largest of all garden plants, with (forms) such big branches---" LB, "one of the largest of plants, with long branches---." Only the Amp Bible writes, "greatest of all garden herbs---" All interpretations are much the same for both Matt and Mark. I guess the point is that everything is a matter of human interpretation and and it seems to me that what is most important is to gain the SPIRITUAL import of the Scriptures of which there is only one. It is not mine or yours but God's given to each of us in the quiet of silent prayer. I agree that word definitions are MOST important and useful in discovering the spiritual aspect of the Scriptures, but only to that end.