If one testicle is bigger than the other, is it likely to be cancer?

If growing bigger. If one testicle is growing bigger than the other one, after puberty, there could be a tumor in the growing testicle. A doctor's evaluation is needed. A ultrasound scan can be done. If a child developed one small and one bigger testicle, but they stayed the same size after puberty, they are likely ok. However, if the smaller one then grows to match the bigger one, a doctor can check for cancer.
Benign or malignant. Scrotal ultrasound will give the answer, unless there is an obvious hard lump which is always considered cancer until proven otherwise. Good luck.