How did the gas giants become so large?

Similar to "ordinary" planets, gas giants began life as slowly accumulating chunks of rock and boulders that attracted additional material due to their larger mass.

However, these potential gas giants grew in an advantageous location, where they could accumulate a particularly large amount of rock and where there was an abundance of surrounding gas; the large, massive planets then caused a runaway effect, whereby their gravity effectively sucked in everything around them, intensifying their mass and gravity until everything in the vicinity was drawn in.

Naturally, Jupiter is the dominant planet in our solar system; even among giants, it is a giant, containing over twice as much mass as all the other planets put together. The other outer planets concentrated enough primordial mass to become large due to the "runaway" effect, but they were forced to make due with whatever was left over after Jupiter's dominance.

Gas giants became so large due to their formation process in the early solar system. They formed from the accumulation of hydrogen, helium, and other gases, along with dust and ice, through a process called core accretion. As their growing mass increased gravitational attraction, they began to attract even more gas and material from the surrounding protoplanetary disk. This accumulation continued over millions of years until the gas giants reached their current sizes.