6-max is 9-max where the first 3 EP players have already folded :)

9max players (at lower stakes) are usally tighter than 6max players (don't really know why, as the others said, it's the same game when first 3 EP players fold). As a result, u could be more agressive in blind vs blind spots and in late positions.

It's not quite as simple as the "first 3 players folding".

The biggest difference to me is...
You'll be posting the blinds more often which is why the games are less tight than 9 max games. So for every six hands at 100nl you'll pay $1.50 while at 9 max you see 9 hands for that $1.50.

This also leads to you playing in the blinds more often but also being on the button more often as well. So if you've got huge leaks in blind play or opening ranges etc it'll be more pronounced.

Stealing becomes more important again because of less players and cost of blinds every 6 hands. You can't really just sit back and wait for the nuts, though I do see a ton of players doing this.