It makes it sound like a big episode but so did the series 2 finale with the Dalek and Cybermen 'war' (which we got about one scene of, and millions of pointless CG Daleks which do NOT make an 'epic' finale).
The fact is, 44 minutes isn't long enough to deal with something this big, especially if it's all wrapped up and no cliffhanger (which it probably will be because they'll want the Christmas episode to be as standalone as possible). We will be forced to endure another last-minute Deus Ex Machina ending where Martha throws a switch that conveniently makes everything all right again.