Though this may interest no one but myself, I found a remarkable explanation for just this very question.  The following is a reader response (to another reader's comment) on why the samurai depicted in The Seven Samurai appear to be overacting (taken from the reader mail section of Roger Ebert's website:

"... American and European film acting is heavily influenced by the realist movement of the 19th century as exemplified in theater by the plays of Anton Chekov. The realists wanted their work to portray slices of real life (hence the name) and in theater pushed their actors to act as naturally as possible and still be performing a piece of theater.

Japanese films are heavily influenced by Kabuki theater. Kabuki theater as it developed in Edo (modern Tokyo) is known for its stylized dramatic forms, exaggerated make up, and over-the-top performance of characters based on broadly drawn archetypes or stereotypes. It is worth noting that Tokyo based Shochiku, Company, Limited the oldest continually operating film studio in Japan and Akira Kurosawa's employer began in 1895 as a Kabuki theater. So the actors in "Seven Samurai" aren't over acting they are simply acting in the style with which they were most familiar."

- Frank B. Chavez III, Hayward, CA