If they suddenly shift to a 6-person party - given that all encounters are presumably intended for 4-person parties - how can this not affect encounter design/difficulty? Surely encounters will be too easy so you'd need to rejig them?
Each character in a 6 person party will have 4/6 as much exp as characters in the 4-person party. Game auto-balances itself. If this doesn't do a perfect job of auto-balancing for different party sizes, Larian can adjust the exp formula for differently-sized parties.