Small as it is, the sparrow has all the vital organs.

解释 ( paraphrase )
麻雀的身体虽然很小，但是它的五脏六腑是齐全的。
这条谚语告诉我们：尽管某种事物很小，但是包含的内容却样样齐全。也说明小事物同样会有代表性、典型性。

This proverb reminds us hat something may be small in size, yet it may still contain all the necessary parts. The meaning is that something small may represent things much bigger and general.

俱：全，都。
      All.

同义词 ( synonym )
麻雀虽小，肝胆俱全

造句 ( example )
你别看我们那个房子小，“麻雀虽小，五脏俱全”。