There are bigger ones that emit more light. But they are intended for use in darkness when your vision has night adapted (wait in the dark for a few minutes). You won't see them glowing in room lighting, if that's what you expected. If you want more brightness there are flashlights with locator LEDs. Those consume some battery power so you eventually have to recharge the light, but it is a nice feature. Come over to!flashlight@lemmy.world if you want suggestions.
It's a GITD marker that lets you find your thing in the dark. It's also used for markers on watch dials and gun sights. But in at least those tiny sizes, it's not a flashlight substitute or anything like that. You could add a flashlight to your keys, so the tritium helps you find the key ring and then the flashlight gives you more light if you need it.
It looks like the style from glowrhino.com. That said, I've never felt a need for a tritium marker on my keys. I have one on a flashlight, which seems more useful. You can get bare vials at mixglo.com though they are expensive there.