Why does the size of the universe change?

I limit myself to just changing.

In our universe, matter is the most important thing.

A particular kind of matter experiences dimensional changes as a result of

distinct occurrences in nature.

In general, they are contraction/disintegration, absorption/expansion,

as well as growth and incorporation.

Consequently, the area occupied alters, and the proportional space within

in between modifications.

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'.

The size of the universe doesn't change in the sense that objects within it are physically growing or shrinking. However, the observable universe appears to be expanding due to the expansion of space itself. This expansion is described by the theory of the Big Bang.