The issue is all to do with how much glide you require for your weight to catch the wave you are riding. (Small large fat or dredging of its dial)
If you ride the smallest lenghted sup that is under volumed , you will always have a pretty late take off and find the draw off the bottom way harder to contend with, even when glassy , as SUP boards sit on the surface.
This combined with a high center of gravity and wind resistant body requires you to paddle as hard as you can to gain forward motion , causing the board to pivot.
But actually in many cases you get drawn backwards ,this makes the short sup board more unstable, often it will / can squirrels side ways on a paddle stroke for a bit before the fins / rail bites,
You are also requiring better quality surf and conditions ..............
When your in this situation on a surfboard you are able to sink the tail into the face and a spring board forward kicking your legs and hands all with a lower center of gravity,
If your constantly, falling off, missing waves , finding your self out of position in upto 15knots with some chop , and you can not chill axe in the line up, out of the surfers face and allowing the surfer in front of you to go , your boards to small dood and why bother
You gotta get your chicken leg wobbles under control dood

your cracking me up