Observations from other forums suggest the 8971 MC movement is actually a Jaeger-LeCoultre 846/1 manual wind caliber and yes, this would be quite small for the case. The movement is neither round nor square, but ellipse-shaped. To get an idea of its proportions, if you look at the rectangular minute
To me it's a near perfect example of Patek's perpetual calendar design. I love the applied romans and that retrograde gives it just a little bit more intrigue and class. Size is great and I've always been a sucker for a silver dial. I love it!