Ever been to a barbeque? Someone starts the charcoal using too much lighter fluid and giant flames erupt. After the flames are going, more fuel will make the flames bigger, brighter, and hotter.
Ever play with black powder? The more you use, the bigger the bang. This is always the case.
Nuclear weapons are the same way. The bigger the core, the bigger the explosion. However, you can make a bomb so big that it wont explode. That's right. You give it soo much fuel that it would rather burn slowly, over billions of years.
Cue the sun. The fantastic giant bomb that never explodes. It's magical properties allow it to maintain unimagineably high pressure, heat, and fuel supply (3 things that bombs love) without ever exploding. Just slowly burn, you magical monster you!