So, a little experiment I did with a few students.
We planted some small potato onions into one tray and labelled it 'small' and some big ones in another tray and labelled it 'big'.
Then I took both home to my greenhouse and left them there.
As you can see, the big ones on the right have more green shoots than the small ones on the left.
This means that the big ones will grow more actual potato onions but each one will be smaller, and the small ones will grow less potato onions but each will be bigger.
Works for all alliums, including shallots and garlic.