It may be because pistons can move up and down to keep the system at a constant pressure (isobaric conditions). If you heat up the system that is initially at 1 liter, the gas molecules move around a lot and exert more pressure on the walls. This moves the piston up until pressure equalizes with the exterior again. The new volume is greater than 1 liter. Since volume is correlated with mean free path, an increase in volume means an increase in mean free path.