Reasons: for a smaller wave height and given rider weight, the board is not going to get on a plane unless the surface area is large-enough. for smaller wave heights, planing hulls require you either to go longer or wider or both, therefore you need more planing surface. If you don't go fast-enough to plane due to lower wave energy, your speed is going to be proportional to the board length given by
Hull speed calculator is a simple calculator that determines a vessel's hull speed based on the length of the vessel's waterline.
www.omnicalculator.com
MOAR foam for a given length/width of a board will not acheive this - it will only float you better before take-off. It has little effect on planing characteristics and hull speed for a given lenght.
You just need a longer board for small waves - those high-price trendy grovelers depicted in videos with non-groveling waves will not help.
I have been drinking.