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Whether it be the moon (especially when full) or tonight, as Mars is closer than it has been in decades, it appears that these bodies are larger when close to the horizon than overhead.

Is this an optical illusion (i.e. the actual appearance is no larger), or is there some refraction or other effect that actually makes the appearance larger?

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No, it's an illusion. Probably the ancient one. Simple experiment you can do is, set grid on telescope, measure the angle subtended when moon is at horizon and when moon is atop. You will see angle subtended by moon is same, hence the size are same. It's merely an illusion.

For more details: https://en.m.wikipedia.org/wiki/Moon_illusion

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  • $\begingroup$ PS: And same goes for other celestial Bodies. $\endgroup$
    – Kartik Chhajed
    Jul 20, 2018 at 2:33
  • $\begingroup$ PS: Or it could be beaver moon. $\endgroup$
    – Kartik Chhajed
    Nov 23, 2018 at 12:57

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