Speck and beam/log in the eye
Here’s what it means
A hypocrite focuses on another person’s small fault while ignoring a much greater fault in himself.
Why the Bible says it this way
The writer deliberately uses very strong language to make an important point. The point is serious even though the wording is not meant literally.
A helpful detail: The absurd size contrast makes hypocrisy unmistakable. Jesus still says to help the brother after honest self-examination.