That's interesting.  How much SUP experience do you have? Did you learn on flat or choppy water, and how long did it take to consistently ride on the foil? I read that body weight + 30 litre is about right for a beginner with good board skills which would be 120 to 125 litre for me.

I did have kite foil , wave-SUP and windsurfing experience (but I'm not very talented )
It took me 1 or 2 sessions to ride on the foil. This sport is not difficult to learn.

The thing is, as soon as your wing is pulling a bit, the extra volume is useless.
You just need a bit volume while lifting your wing and while getting up on your feet.

I did start on a choppy lake. After like 5 sessions I even managed to ride in waves.
I don't see the advantage of more than +20l.