I dont know how true it is.. or if its an urban myth...
The solid rocket boosters have to be transported by train from where they are manufactured.. but have to travel through a rail tunnel that is something like 100 years old.
When the tunnel was built, it size was determined by the width between the rails.... The US follows the British gauge for their railways...
As we all know

the British rail system gauge is the same as the width of a 'cart'.. introduced by the Romans. That width was determined by the width of the rear end of a horse...
So.. the shuttle 'size' is directly related to the width of a horse's a$$
