-ок is one of the diminutive suffixes, check here

Suffixes of Russian Nouns - Examples and Translation of Russian Suffixes

Жучок is a small bug.

Лист - листок
Луг - лужок
Сын - сынок

When compared to other diminutive suffixes this one is considered to convey not the full meaning of diminution but partial

лист - листок - листочек
луг - лужок - лужочек
сын - сынок - сыночек

If you want to learn more, search for "diminutive suffixes" or "уменьшительно-ласкательные суффиксы" as they are called in Russian.