Look at any old photos and it's clear that the crowds were very much bigger in the fifties and sixties.  A good number of spectators may have been locals, I don't know, but nearly every street in Douglas had bikes parked side by side - including the length of the Prom.  The loading/unloading was a pain though and I never understood the tank pumping out regime. The only absentee was media hype which may explain the current perception that the event has never been bigger!