So that it dissolves more slowly.
The lozenge is intentionally designed to dissolve over the course of about 8 minutes. While it might be possible to achieve that dissolve time with a smaller lozenge by making the lozenge extremely hard, such a hard lozenge would have a less consistent dissolve time and would be more difficult to dissolve in water or break apart.
So that it tastes better.
A smaller lozenge would concentrate the flavor of the herbal extracts. As such, it would be less pleasant tasting.
So that it’s more effective.
A larger lozenge results in more of the Banjo ‘solution’ – the fluid mixture of saliva and dissolved Banjo – resulting in better distribution throughout the mouth, better absorption and better efficacy.