Hi Cedric, it's a bit more complicated when you bring the school boards in - the better ones are usually longer with a straighter outline - so that beginners can stay upwind/this shape works better with a daggerboard.
For Intermediate boards of 130-150L, compared to older generation of boards, they have become thinner and wider, like our Gecko. And even more extreme is the slalom race boards, which are really thin and wide. Width helps planing, but if the boards are too wide, then you need a much more active pumping/planing style, to breach the displacement mode into full planing, also bigger sails - this is why it's not a problem on slalom boards/raceboards, as riders use very big sails and have the skills. But for intermediates, they will not use big sails and will have a more passive planing style, basically waiting for the wind to lift them up - so if the board is too wide, this will not work.
Re your question, in terms of Gecko vs Viper - here it's a combination of thickness and length which is making the difference, with the Viper mostly used in displacement mode, not in full planing - therefore a thinner, more compact shape is needed, all that volume is not needed once you are up and planing. Also the length is not needed once you are into planing windsurfing. Hope that helps to understand it.