It's an interesting experiment, but I don't trust the conclusion. Automatic translation still tends to be fairly literal, rather than idiomatic. I have a suspicion that this tends to increase the average message length. An interesting experiment would be to translate English tweets into those various target languages, and compare the two. If my hunch is right, the text will get bigger in both directions. The ratio between the increases may be a good measure of the information density of idiomatic expression.