I honestly don't believe the swings are really that different. There are two factors that make the swings appear larger:
1. You play 50% more hands per hour in a 6 max game, so the
hourly swings are larger even when SD/100 is not significantly larger.
2. Many people move to the 6 max games from smaller games (say 2/4 full to 5/10 6 max). Since the higher limits are tougher, their win rates are lower. Lower win rates mean larger and more frequent downswings.
And this is coming from someone who has experienced huge swings at 10/20 6 max. The reason, though, is that my win rate is small at that limit, so the swings are expected. It's not because the game is short.
But Jeff W says it better than me.
Link
That is a very interesting thread, btw.