I went into Edit > Options > Corkboard and changed the corners to square and the label indicator to pin. When I did, the notes on the corkboard got smaller. Even when changing the corners back to square, and the label indicator back to corner mark, they have remained small.
In the bottom right corner of the Corkboard View there is an options button (looks like four tiny index cards on a footer) that allows you to adjust a few visual options for the cards on the board (Ratio, Card Size, & Spacing). Make sure the “Use Small Font” box is unchecked.
If this is not the problem, I’m curious to know what your card fonts are set at in the Options menu.