Who said they were daggers? Please cite in-game text that explains that NINs only wield daggers.

Clearly, if a PLD is able to wield anything between a gladius and the Curtana, which is arguable something like a broadsword, then its totally reasonable that NINs would have the ability to wield both daggers and long-knives, which I think any of those fit into.

Some of the weapons are going to be bigger than normal and some will be smaller to add flavor to the world.