Having boards from 85 to 175 cm, 6 different sizes, but only considering pocket boards in this thread:
Really short pocket boards cons -
Nose scoop not possible, giving the first 4 cons here.
Can not be ridden on the surface really.
IF they should be ever so slightly ridden on the surface, the angle should be set so you stand awkward with front foot bent.
Extremely difficult to waterstart strapless, if trimmed for a natural stance, straps much better of course.
Any touchdown in chop/waves can lead to a crash, both going straight and in transitions and jumps.
If seaweed, you WILL touchdown often, no matter how skilled, not fun without a snub nose - but okay, doesnt matter as you have to stop anyways...
You have no chance of riding safely in lulls if not sufficient to foil, so practical low end is hugely compromised.
You will risk huge crashes when you land from high jumps nose first (in order to foil away) and get the timing off just a bit.
You have no flotation so when you drift ashore you can not sit on the board very well, or not at all.
Dragging out in onshore marginal winds is more difficult, harder, if a thin board no volume at least.
Really short pocket board pros -
Smaller so packing is easy.
Swing weight "mentally" is much better, no doubt.
Some think it is cool (but others think it looks odd so can be a pro as well as a con)
With a small or ultra small light board swing weight differences can not really be felt in my experience.
Nor can the difference when board dragging through waves- small or ultra small not any difference.
Weight difference can also be minimal, as you dont need strength nor stiffness in the nose really, only between your feet.
I love really small/short pocket boards, but only for the fun of "just doing it", as no practical advantages at all for me.
Maybe I am just too lousy at foiling, I know, but I suspect many to be at the same level and not like Horst's and this could be one reason why we have so different views
But in general, going to the extremes is either for "just doing it", or a smaller niche IMO, no matter what we are talking about, could also be super small or super big kites or anything else - doesnt work well in the extremes, but some like that