From a Sponichi interview with a Mizuno spokesperson:
There appears to be an increase in home runs, have there been any changes to the ball?
Nothing has changed from the ball that was used last year or the year before that.
Maybe there was a change within the set limits.
There have been no changes, that includes the coefficient of restitution.
A number of players have said the ball feels different upon contact, compared to last year.
Why they are feeling that, we are not sure.
There have been no changes to the width or height of the stitches either?
Nothing has changed since the introduction of the ball.
What about the type of leather used?
All we know is that we are buying the same stuff.
Source: Sponichi 4/16/2013
Sponichi has also posted a few other articles that include observations made by players and coaches. All of them more or less think the ball is traveling a bit more this year than previous years.