Well, the second secret is having a very rigid and precise interface between the watch case and back cover so that the thin gasket remains under the correct amount of pressure everywhere around the joint. This has been optimized at the small size of a watch.
On a larger object, this level of precision is harder to maintain. Due to spacing between fasteners or other flexing/distortion of the body, the gasket could be overly compressed in some places and loose in others...