Why is Jupiter the Biggest Planet?
Firstly, during it's formation, Jupiter's foundation was likely the largest of any planet, giving it the gravitational advantage to collect more mass over time.
Secondly, during its formation, Jupiter was far enough from the Sun, as a result the Sun's gravity didn't attract as much matter away from Jupiter.
Lastly, Jupiter captured so much hydrogen and helium gas from the Solar Nebula that it ended up with 300 times Earth's mass.
https://science.nasa.gov/jupiter/jupiter-facts/
https://www.space.com/7-jupiter-largest-planet-solar-system.html