Why do larger planets tend to mostly be made up of gases?
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It’s a bit of a chicken-and-egg problem.
If you’re a planet, you need gravity to attract stuff. The bigger you are, the harder you pull. But working the other direction, the lighter something is, the weaker you will pull on it.
The really heavy bits like metals and rocks will tend to find themselves and collect into piles pretty quickly. But the lighter stuff will be harder to rope in, since it’s just really hard to pull on in general since it’s so light.
You’re gonna end up with a lot of sort-of small rocky chunks quite easily. Countless many of them very small asteroid-sized, a few of them planet-sized. Of them, only the ones that happen to get big enough by random chance are going to start collecting the lighter gases floating around. And once that happens, it starts to snowball. More gas is more mass, more mass is more gravity, more gravity is an easier time pulling down more gas.
Planets like Earth aren’t really big enough to have been able to do this. Earth has an atmosphere, yes, but as a proportion of its size, it’s *barely* there. If Earth was an apple, the entire atmosphere would be thinner than the apple’s skin. Most of Earth’s atmosphere was not captured; it either rode in on meteors from the far outer solar system where those gases would have been frozen solid in the cold, or generated from solid or liquid materials through chemical reactions.
Planets like our gas giants, however, probably started out kind of Earth-like in makeup, but by random chance, got bigger. Big enough to start clawing up that really light-weight hydrogen and helium gas that was everywhere that planets like Earth were too small to hold onto. After reaching that critical size and shoveling it up, they snowballed into the giants we know today.
Even the Sun itself likely started out vaguely Earth-like at some point, as a heavy chunk of rocky matter. The Sun formed more or less like any other planet did, just stuff clumping together, heaviest stuff first, then slowly clawing in lighter and lighter stuff. The only thing about the Sun that makes it different is that it was the lucky one to get biggest *first*. The runaway snowballing of the lighter gases made it get really big really fast, faster than any of the others, up to the point where it finally ignited its nuclear furnace and started to shine. That shining whipped up solar winds, which blew away all the extra dust and more or less halted the growth of all other planets, freezing them at more or less their final sizes. From that point on the only thing planets could really do to grow was crash into each other and merge.
tl;dr: the gas giants are just the phenomenally lucky ones.
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