Also, on a pony saddle the leather can be "lighter", slightly lower quality or just plain a narrower strip. Over time, the left side stirrup leather will stretch as it seems to be the only side kids mount from when learning &/or small enough to use the smaller saddles. You may have to add holes to be able to adjust the stirrups to be balanced from side to side. When the horn is centered over the withers, both stirrups should hang at the same height (unless specifically adjusted not to, of course).
Once sitting in the saddle, I wanted the bottom of the stirrup to hit the ankle bone when the boot out of the stirrup and hanging relaxed. Again, you can adjust slightly longer or shorter... A lot of the pics of our daughters show their stirrups much too long or too short (sheepish grin) and while trying to get a deeper heel down on one of my students her stirrups seemed to be always too short!