For the uneducated observer, of course the Sun looks like a ball in the heavens that is hanging not too far in the sky. So, why blame him or the people of his time to believing that if the Sun comes down, it will look a bit bigger but not that big!!
Sources of the Hadith:
https://sunnah.com/riyadussalihin/introduction/402
https://sunnah.com/muslim:2864
For more silly hadith that make no sense, read this link:
https://wikiislam.net/wiki/Qur%27an,_Hadith_and_Scholars:_Remarkable_and_Strange_Islamic_Traditions