Question

the sun is not the biggest or the brightest star.why does it appear to be so

Answer 1

Though sun is not the biggest or brightest star, it appears so because it is the nearest star from us.
Since the distance is very less, it appears bigger and brighter.
As the distance of other stars from us increases, their brightness and size appears to less.

Answer 2

the sun is not the biggest star but is nearest star from us. so it appears bigger and brighter because as the distance increases, the size and brightness decreases and when distance decreases, size and brightness increases.