Design variations
Bike helmets are properly padded for bike accidents and equestrian helmets are properly padded for riding accidents.
Horseback riders are more likely to fall so that the back and sides of their heads are hit rather than the front like cyclists, so the helmets are designed to absorb more back and side-impact and little front impact.
Motorcycle helments are heavier, hotter and reduce peripheral vision/ hearing.
Hard hats are also made to prevent injury to the top of the head from falling objects and debris.
Ski helmets are geared to protect the back of your head as well as the top, can get quite hot.