UK Mooncup size A (large)
only 13ml to holes
Lunette size 1 (small)
19ml to holes
Diva size 2 (large) - right-side out
27ml to holes
Diva size 2 (large) - inside out
26ml to holes
Tampax tampon size Super
Dunked it in a glass of water, held it up until it stopped dripping, squished out the water and measured what came out.
max capacity - appx 23ml (Water and blood have different viscosities so take that into consideration.)
EDIT -- NEXT DAY:
I measured another Super Tampax. Using the syringe and a sandwich baggie for both containment and a little "wall" pressure, I measured how much water I could put in before the tampon stopped absorbing.
It was 24ml.
Take all these numbers with as many grains of salt as you think they require.