Act size don't have to be uniform, consider Baldur's Gate II, it had "chapters" but its first act was much bigger than any other segment of the game. What three acts might mean though is that most of the story wont go very far from Baldur's Gate.
That. For exemple, in D:OS2, the first act was big on content, but the 2nd act was huge. They don't have to be all the same size.
And of course, don't forget that we have yet to see all of Act 1. I don't think we have seen 1/3 of the game so far.
