If my very quick research is correct,

6 x 6mm heatpipes have a total circumference (fin contact area) of 112.8
4 x 8mm heatpipes have a total circumference (fin contact area) of 100.4

BUT

6 x 6 have an internal area (flow area) of 169.8
4 x 8 have an internal area (flow area) of 201.2

Circumference = pi * diameter
Area = pi * (radius squared)