What connector standard is used for aviation headsets?
It's for the microphone (the standard sized one is for the head phones). They are different size so the pilot knows to try the other hole if it doesn't fit.

From wikipedia:


  Commercial and general aviation (GA) civil airplane headset plugs are
  similar, but not identical. A standard 1⁄4 in monaural plug, type
  PJ-055, is used for headphones, paired with special tip-ring-sleeve,
  0.206 inch diameter plug, type PJ-068, for the microphone. On the microphone plug the Ring is used for the microphone 'hot' and the
  sleeve is common or microphone 'Lo'.