Oh my. That's a big one. Do you think he's compensating for something?
Watching the movie, you might notice the sheer size of the weapon Kikuchiyo is carrying around. It's a colossal two-handed sword (the formal Japanese term is "odachi"), and for most of the movie, he really has a hard time controlling it. It slips and slides around his body when he moves, tangling up his feet and generally making him look ridiculous. He can swing it at least, but considering how much reverence and devotion samurai place on their weapons, he's definitely not giving it the respect he should.
That's in keeping with his character—a farmer's son trying to pass himself off as a samurai, instead of someone who actually trained with the weapon he now wields—and it also demonstrates how fluid class lines have become in this world. There was a time that if he even touched a samurai's sword they'd take him into the nearest square and cut his head off. But now, with central government in disarray and the whole country taking a "let's do what the heck we want" attitude, he can get away with it.
In that light, the sword becomes a symbol of the position he's usurped, a way of showing us what he's not, but what he'd do anything to achieve. Yeah, it makes him look ridiculous, but he can still cut bandits' heads off with it… and at the end of the day, that's what the darn thing was made for.
In fact, in its own weird way, it embodies part of the samurai code even as it points out how goofy this guy is for carrying it. A samurai's sword is supposed to be an extension of his soul. Fathers handed their swords down to their sons and sometimes, the sword would even be present at a birth in a samurai family, to make sure it knew who its master would be. A samurai never let it out of his sight and in fact there were only a few instances when anyone else was allowed to touch it. It truly was a part of him.
And this thing is, for better or worse, a part of Kikuchiyo. It's big and unwieldy, like him. It makes him stumble and crash around a lot and in his hands, it doesn't look like he can even unsheathe it, let alone hurt someone with it. But when he manages to get it out, he knows how to use it, and cuts down his share of bandits with a surprising amount of effectiveness. Whether he should be carrying it or not starts to become irrelevant. It's his, and the rest of the world will have to deal with that.
By using it the way he does, he actually succeeds at his goal. Just as the sword becomes an extension of his soul, so too does it justify his pretense at being a samurai. He fakes it 'til he makes it, and the sword helps him do it. We see that at the very end, when it sticks out of his grave alongside the three swords of other, more "legitimate" samurai. Kurosawa's telling us here that he's a samurai in every way that counts, even if his stupid-big sword doesn't make him look at way at first. More signs from the movie that things aren't as inflexible as they once were, and that, if they try hard enough, people in this world can remake themselves into who they want to be.