What stars/planets are bigger than the Sun?
There are no planets bigger than the sun. Stars larger than the sun include stars farther up in the main sequence, giants, and supergiants.
Bodies of such large size cannot remain planets because their gravity will cause them to fuse atoms and they will simply become stars.
Giant and supergiant stars are larger than the sun because they are a different type of star. Fairly simple. Stars in the main sequence on the HR diagram follow a proportional path of brightness, temperature, and size. Thus, stars on the hotter/brighter end are also larger.