2 Corinthians 10:10

For his letters, say they, are weighty and powerful; but his bodily presence is weak, and his speech contemptible.

The saying that his letters are weighty and powerful but his bodily presence is weak and his speech contemptible was the hostile charge made by his opponents at Corinth, who conceded the force of his letters while disparaging him in person, and it is their criticism rather than a true assessment of his ability.
The charge about his diminutive body and contemptible speech was false, for he was certainly no mean speaker, and the force of his eloquence brought the greatest men under convincing power as they listened, with men at Lystra mistaking him for Mercury come down in human form and thousands of Jews and Gentiles in many places believing and being converted through his preaching.
His unimpressive, lowly appearance in person was connected with his humbling himself to do manual and mechanical work, and that humility was mistaken for weakness.

He deliberately refused to present himself as a fine orator, debater, deep philosopher, or ruler over conscience, avoiding flourish of words, boasting style, and pompous show of science or eloquence as he had learned from the divine Teacher, and instead presented a divine revelation and spoke divine wisdom, so that his success came not from himself but from the power of God upon him, with angels making the Word effective.
His letters were weighty and powerful, carrying a powerful testimony about his experience, and they partly supplied for his absence from the churches, being read in the churches as God's word delivered through his obedient servant.
His lack of outward rank, commanding physical presence, and persuasive speech did not leave him powerless, for he carried a commission from Christ and his weapons were mighty through God to overthrow sin and Satan.
In the same context he defended the authority God gave him for building up the church, resolving to act in person with the same decisiveness he showed in his letters, yet exercising that authority with sorrow and care for opponents rather than with the coercive force he had once wielded.