To put it in chemical explosive terms, let's say you have a two part explosive - fuel and oxidizer. If you mix them very well you'll get complete combustion and more energy released. Mix them coarsely and it'll be relatively weak.

There's only a tiny amount of time in which a nuclear bomb can support a nuclear reaction before it stops working. After some point it stops reacting because it's just too large on account of exploding.

Tritium helps it get to a fast start, make more neutrons and burn more fuel within that brief window. Some amount gives the best efficiency. Too little and more of the fuel will survive without releasing its nuclear energy. Too much and, well, you probably run into interesting problems with it exploding too quickly or something like that.