Here's why:

* Observable Universe: The observable universe is the portion of the universe that we can observe from Earth. It has a radius of about 46.5 billion light-years. This vast region encompasses all the galaxies, stars, planets, and other celestial objects that we can currently detect.

* Galaxies: While galaxies are incredibly large, even the largest known galaxy (IC 1101) is only a fraction of the size of the observable universe.

* Stars: Stars are much smaller than galaxies, even the largest stars like VY Canis Majoris pale in comparison to the observable universe.

* Planets: Planets are much smaller than stars, and even the largest known planet, HD 100546 b, is dwarfed by the observable universe.

Therefore, while other celestial objects may be large on their own, the observable universe encompasses them all and dwarfs their size.