I copied the file to my server, viewed the hex data for the first 512 bytes and noted that the first two bytes are ascii
II which I recognized from previous experience as the intel byte-order flag for TIFF.
I gave it a TIFF extension and my desktop OS created a preview thumbnail automatically which matched the contents of the sketch.
So: it is a TIFF file with layer support.
I altered the canvas size and it opened just fine within the Sketchbook program (when placed in the proper directory and named appropriately).
I did not test compression methods (LZW RLE JPG etc) nor did I probe the size and memory limitations on the android device.
Note that this means you can easily "back of the envelope" something on the train etc and then bring it into your desktop environment without having to export and lose layer compositing.