Why do the scenes towards the end of Macbeth get shorter?
I always liked the short scenes in Act 5. They alternate between outside and inside the castle. We see the developments outside the castle as Malcolm's forces gather at Birnam Wood, and are ordered to use tree limbs to shadow their numbers. Alternately, we are shown developments inside the castle as Macbeth goes from being contemptuous of the reports about the English forces to clinging desperately to the witches' prophecies, as he realizes that his men are deserting him and his wife has committed suicide. The suspense increases as Macbeth slowly comes to know what the audience has already witnessed in the previous scene: Birnam Wood is coming to Dunsinane. This back and forth movement increases our anticipation of the fulfillment of the prophecies and the inevitable showdown between Macbeth and Macduff, which does not disappoint.
Concerning Shakespeare's Macbeth, I haven't done an analysis of scene length in Act 5, and I have never read anything that comments on this, but in general scenes are sometimes shortened near the conclusion of literary works to increase intensity. In novels, chapters may be shortened, and in plays, scenes.
Moving from scene to scene quickly can give the impression of speed: the more scenes, the more intensity. Events seem to occur one after another, rapidly. This would be desirable as the play drives toward Macbeth's death. The speed of events should increase, the closer the events get to Macbeth's death.
Also, don't discount the practical issues involved. Shakespeare has plot lines to finish in the final act. A great deal happens, and perhaps the shorter scenes just reflect that.