Exploratour - Evolution of the Solar System
As shown in this picture, while they were forming in the solar nebula, the core of the planets-to-be drew material to themselves from the cloud of gas and dust around them. The bigger planets-to-be were able to draw even more material unto themselves because of gravity.
Because of it's position in the solar nebula, the proto-Jupiter, was able to draw an enormous amount of gas unto itself, and become the biggest of the planets.
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