Supposedly, the original German pilot watches were 55 mm to accommodate the larger pocket watch movements of the day (which were probably more robust, more accurate, and maybe had longer power reserves than existing wrist watch movements).
Of course, size helped legibility, too. I notice reading a watch is harder when being bounced around in the Speedster. Probably even more of a challenge when a squadron of Spitfires is shooting at you.
They were also worn over flight jackets.