The diameter and the focal length (fore-and-aft position of the bulb relative to the reflector) both have to be correct. The reflector is shaped to "look" at a filament in a very specific position. If the focal length is even a small bit off, the reflector can't gather and amplify the light from the bulb, so you'll get a useless little dot of light from the filament viewed through the lens, but not the big, bright, fully-lit lamp area you need to alert the dillweed behind you in the middle of a very important, urgent phone call that it's time to stop.