They actually tried to explain the glasses once. It was in an issue I read when was a kid, and I couldn't get my hands on it now. As I recall, it started with somebody drawing Clark Kent's portrait, and Kent realizing it didn't look like him. Soon he understood that everybody saw him as slightly older and thinner than he really was.
Good old Ma Kent had to remind him why it was so:
Back in his Superboy days, our hero quickly realized he had to keep people from recognizing him in his civilian identity. He needed something simple, and glasses seemed like a good idea.
Super-glasses, of course, would be even better (in those days, everything had to be super, including Krypto, the super-dog). Young Clark Kent still had the capsule in which he'd arrived to Earth as a baby. He used the Kryptonian glass from one of its windows to make his glasses.
As it turned out, the glasses gave him mild hypnotic powers. They made people see him a little differently without any conscious effort on his part. This worked out well overall, but led to occasional puzzled comments along the lines of: "You know, you look really muscular from behind". This may also mean that the glasses are indestructible.
That was long ago, and DC Comics has made many revisions since. For all I know, maybe the secret origin of Kent's glasses has been retconned out of existence.
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