Why does the size of the universe change?
There are many theories on why the universe is expanding, however one of the more common ones is that after the Big Bang, and explosion from the central point, the particles are still all moving away in all different directions, and so we continue to move 'out'.
I confine myself to change only.
What matters most in our universe is matter.
In a specific type of matter, dimensional changes occur due to
different natural phenomena.
Broadly, they are absorption/expansion, contraction/disintegration,
Accordingly, the space occupied changes and the relative space in-