Even after reading the FAQ (which is excellent: https://www.amsmeteors.org/fireballs/faqf/) I'm not quite clear why a meteor would be a fireball versus not being one. Is it simply about size, or something in the composition?
Mostly size. Ordinary shooting stars are grain-of-sand sized, and what you see is actually the superheated air that they displace coming in at 10+ km/s. Something slightly bigger, like a rock a few cm wide, can cause a blinding fireball.
Speed also counts. It can range from 8 km/s (a "stationary" object, relative to us, falling into Earth's gravity well) to about 70 km/s (something in a retrograde orbit in the Earth's plane). That's a factor of almost 80 in energy to be dissipated
> Meteors are the result of the high-velocity collision of meteoroids with Earth’s atmosphere. A typical visible meteor is produced by an object the size of a grain of sand and may start at altitudes of 100 km (60 miles) or higher. Meteoroids smaller than about 500 micrometres (μm; 0.02 inch) across are too faint to be seen with the naked eye but are observable with binoculars and telescopes; they can also be detected by radar. Brighter meteors—ranging in brilliance from that of Venus to greater than that of the full Moon—are less common but are not really unusual; these are produced by meteoroids with masses ranging from several grams up to about one ton (centimetre- to metre-sized objects, respectively).
I think if you consider light pollution around populated areas and the fact that most people don't spend much time looking at the sky carefully, most of the shooting stars actually noticed by most people are larger than sand, probably more like tiny gravel than sand.
It is just about the brightness. Answer number 1 in the FAQ states:
> A fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation.
Or was your question why are some so bright? That is mainly a function of size and speed.
Meteors are very small. The bright light comes mostly from the molecules of atmosphere in front of the meteor being compressed, heated, and ionized[0] - essentially from the friction of the high-speed meteor entering the atmosphere.
I guess if the meteor was a ball of pure magnesium, it might glow brighter, but there isn't so much oxygen up at that height, so my no-calc-gut-feel estimate is that this would contribute orders of magnitude less brightness than the glowing atmospheric molecules.
To the extent it's about composition it's probably mostly about how long the meteor holds together so it can retain velocity and the surface can stay hot enough to glow. A bunch of small pieces decelerate faster than one big piece. Compositional changes would only change the color somewhat but likely not enough to be worth commenting on.
