Matthew 11:11 Truly, I say to you, among those born of women there has arisen no one greater than John the Baptist. Yet the one who is least in the kingdom of heaven is greater than he.
Imagine a servant fanning a king in his palace in the heat of summer. The king is the greatest in the kingdom, but according to Jesus, the greatest is one who serves.
All of a sudden the first becomes the last because the king doesn't serve anyone. That servant fanning the king becomes the greatest since he serves the king himself, but the lowest servants who aren't even worthy to serve the king become greater than he.
Perhaps that's why Jesus said John the Baptist is the greatest, but the least in the kingdom is greater than he.