Why does a full moon appear much larger rising than at the zenith?

It's because at the horizon, we see the moon next to distant objects we know are large. At the zenith, we see how small the moon is compared with the vast expanse of the sky.

Depending on atmospheric conditions, there can be effects of refraction, but almost always they're small compared with what happens inside our eyes and brain.

Here's an experiment to conduct yourself to measure the size of the moon.