Why are planets around stars so large?

Planets around stars can be large due to several factors, including the amount of material available in the protoplanetary disk from which they form. Larger planets, particularly gas giants, can accumulate significant amounts of gas and dust, allowing them to grow rapidly. Additionally, their size can be influenced by their distance from the star; those in the outer regions can attract more hydrogen and helium, leading to larger sizes. Lastly, gravitational interactions with other celestial bodies can also play a role in their growth and retention of mass.