6 is bigger. It's a measurement based on the idea that you draw the wire more times the higher it goes, so it's thinner. He's saying that the hottub was relatively far away, so they used thicker wire for less resistance therefore less V drop. Now they're using the same run for an application where 6 awg is the appropriate size, as the charger will be much closer to the panel.

https://www.multicable.com/resources/reference-data/cross-reference-awg-to-mm2/

6 AWG is 16mm^2