It seems bizarre at first, but it's not that unusual to see a kunai block a much larger weapon. We've seen that kunai are incredibly sturdy since the first few chapters of the series, and rarely shown broken in half except for the very tip of the blade.
I assume the user's skill also plays a part in a successful block. They are usually strong enough to lock the blocked weapon in place and preventing it from sliding towards a weaker section of the blade (e.g. the point where the blade base and the handle meet), thus lessening the chance of the kunai breaking.
In terms of reality, it's still very much possible to block a longsword with a tanto-sized knife however the chances of the block being successful are far lower than using a larger weapon. Again, it depends on the defender's skill, physical strength and the material of the weapon.