Master and Commander question

It would appear that he considered the existing yard to be too short, and possibly too light, based on the "more like a cro'jack than a true yard" opinion. A cro'jack (really crossjack) yard is the lowest yard on a square-rigged mizzen mast and provides a place to attach the lower corners of the topsail. It does not carry a sail itself, just makes it easier to control the sail above it, so it doesn't need to be as heavy as a proper yard. Jack put enough sail on his undersize mainyard to cause it to break, then replaced it with a stronger yard. The new yard, being longer and stronger, would also allow him to carry a larger mainsail.