For early planing (in the < 10 knot range) width is usually more important than volume (as long as there's enough volume).
A narrow board (<70 cm) with 140-160 liters of volume will not plane nearly as early as a board with a similar volume but 80-100.5 cm wide.
At one point, we actually had "virtual volume" ratings, but that did not reflect the true volume of the board and was confusing to many sailors.
So we have the true "tank test" volume of both wide and narrower boards, but there is no "test" for how early each will plane.
Part of the puzzle here is that wider boards use longer span fins, and in general, much larger sails.
Formula boards are pretty much the acknowledged "earliest to plane" (the Apollo may be slightly better) and they have around 140-160 liters of true volume but are 100.5cm wide.
The Formula Windsurfing class rules stipulate the 100.5 cm width, and 70 cm max. fin depth with a 12.5 m2 max. sail size.
So, for earliest planing, you need a wider board, with a huge fin, and a huge rig.
Once you get above around 15 knots, then width becomes more a liability than an asset as wider boards (without the huge fin and rig of a formula board) do not handle the chop so well and once you get to 20 knots, narrower is often better as it gives you more control.
Hope this helps,