I can't donate blood?

Today I got a call from the NHS asking me whether I wanted to donate my blood plasma to help tackle COVID-19. I had coronvirus around the start of the year so I have antibodies against it. They asked for my height and weight and I said I was 1.63m and around 50kg. They said I didn't have the right volume to donate blood plasma. Can someone elaborate on this?

Donating blood typically requires half a litre of blood to be drawn and short or small people have less blood to give meaning the greater proportion of blood you give, the less blood you have to perform vital functions.

Meaning it is more dangerous for you. Women especially given how much iron they lose through their monthly period and it takes only a couple of days to restore the volume but the actual components of blood (blood cells) take weeks.

Hence why. You seem to be an individual who is to say the least on the light side, no offence!