On Tue, 18 Feb 2014 13:30:28 -0500, micky
wrote:

On Tue, 18 Feb 2014 09:48:17 +0000 (UTC), Judy Miller
wrote:

It always seems I have the wrong diameter headphone jack when I need one.

They started with big ones, 100 years ago. 1/4" As things have gotten
smaller, the big plugs were too big for the little devices.

Since adapters exist, why do they make headphone jacks different
diameters anyway?

I have loads of adapters, but only for temporary use until I get the
right product. Adapters are a pain -- they stick out -- and a little
cell phone or Sandisk MP3 player won't have the power to drive the 2 or
2.5" speakers in a large set of headphones. Even if it did have the
power, it would run the battery down too fast.

Is there a functional difference?

No, except that some headsets need a microphone, and others are
monoaural or stereo, maybe with no microphone. For talking on the
phone, there could be two earpieces but they would have the same sound,
no stereo.
Perhaps the OP is refering to 3.5mm vs 1/8". Very similar size and
usually compatible.