In the first five games in new Yankee Stadium, including the two exhibition games, there have been 25 home runs hit in a mere 45.5 innings. The dimensions of the stadium are the same as the old ballpark but I have heard that the walls are shorter. In addition if you look at the diagram below from WasWatching.com, you can see that the fences are not quite the same. At the the new ballpark the fences are straighter, especially in left and right field due to the new manual scoreboards.
However, despite the slight differences in the outfield fences, I think the reason the balls are flying out of this stadium is the wind. I have not been the stadium yet so I can not say for sure if it is “more windy” than the old stadium but the ball seems to carry a lot to right-center and even left-center. You can not tell me that while they were constructing the stadium they did not have people test the way the wind blows in that area. So, the Yankees probably already new to some degree that balls would carry well in the new stadium.