I've been meaning to do this post for several months now, ever since I noticed this. The sad departure of Maurice Sendak made me decide that today is the day.
My paperback copy of Where the Wild Things Are is 10"x9" (actually it's more like 9 7/8"x9", but close enough). So in each pair of facing pages, there are two times 10" times 9" equals 180 square inches of paper.
The lone illustration on the first pair of facing pages ("The night Max wore his wolf suit and made mischief of one kind") is 5 1/2" x 4 1/8", for a total of about 22.7 square inches, or 12.6% of the total paper. The illustration on the second pair of facing pages is almost imperceptibly larger, at 6 1/4"x4 3/4", or 29.7 square inches, or 16.5% of the total area. I think you can see where this is going... but you still might be surprised at how committed Sendak is to this distinctly mathematical flair:
I probably didn't need to label the Wild Rumpus, but I did anyway. For the entire book, the illustration has been growing and growing, until for those three pairs of facing pages, the illustration fills the entire space, obliterating words (if you don't recall, those pages stand apart from the rest of the book in that there is no accompanying text).
I also labeled the two points where it crosses the 50% line, once on the way up and once on the way down. In both cases, the entire right hand side page is solid illustration, the left is simply black text on a pure white background. The first, which I have labelled "Start of dream?" is the frame where we see Max's arms raised as he prowls through the newly grown forest, accompanied by the text, "and grew until his ceiling hung with vines and the walls became the world all around". If you'll allow me a bit of interpretive license, I would argue that in the pages leading up to where I have marked "Start of dream?", Max has been in that semi-dozing state we often find ourselves in, where images flash in and out of one's head as one teeters between wake and sleep; and the page I have labelled "Start of dream?" is when he falls into a deep slumber, and the vague scenes in his head crystallize into the ragged narrative of a true dream.
The point I have labelled "End of dream?" doesn't require nearly as much interpretation. In fact, I probably could have left off the question mark. This page is accompanied by the caption, "and into the night of his very own room where he found his supper waiting for him", and shows a tired-looking Max smiling and rubbing his head in his room. It is the first post-Rumpus illustration we see that is clearly in the "real" world rather than the dream world. This is pretty unambiguously the end of whatever experience Max was having.
I think it is no coincidence that the "dream" is book-ended by pairs of pages that stand at the threshold where the illustration bursts from the confines of the right-hand page and spills over into the left-hand page -- which, in all of the non-dream parts of the book has been reserved for plain black text on a pure white background. I have to believe this is deliberate.
It's a simple idea, but Sendak was committed to it, and it's subtle enough that I think most readers don't consciously notice it. And yet it adds tremendously to the excitement and other-worldliness of the book. The whimsy grows and grows monotonically, until it begins to impinge on the words, and finally all linearity and verbal thought is obliterated in a frenzy of unconstrained imagination. And just as quickly, the fantastic images shrink and dry up, until we gently touch down back in the world of words and simple material needs and hot suppers.
RIP Maurice Sendak. You were a genius.