Why do most comets have heads with a diameter of about 50 thousand km, although the total range of their sizes

Why do most comets have heads with a diameter of about 50 thousand km, although the total range of their sizes is from 6 thousand to 1 million km?

The diameter of the head is determined by the diameter of the nucleus. There are always fewer large bodies than small bodies. However, small bodies are difficult to observe. The combination of these factors determines the most frequently recorded diameter of comet heads.

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