Most objects in space, including stars and planets, are formed by a process called accretion. Accretion occurs when small particles in space attract each other due to gravity, and begin colliding and clumping into larger particles. After a long, long time, enough particles merge together to become the size of a planet or star. When a star forms, its size is determined by how many small particles there are in that particular region of outer space. If there is a lot of material floating around, a large star will eventually be produced. So, stars can form that are bigger than our sun just by having more starting material to build from.