Ennis' hat is more of a ten-gallon hat than Jack's.
The original Spanish/Mexican hat had as many as ten, possibly more, galones. "Galón" is the Spanish word for braid. Instead of "ten galón hats," English speakers heard "ten-gallon hats." Ennis' hatband is braided while Jack has a solid band on his hat. One galón for Jack, multi galones for Ennis.
i.e., the "gallons" are a measure of decoration, not a measure of liquid volume.
=compliment=
Very informative post Toast. I, for one, did not know that gallons did not refer to volume.
Sandy