Very often the shorter the pony the harder it is to get a saddle to fit, from the small lead rein to the top show jumpers. The pony with some withers and shape, more alike in top line to a horse, is easier and a smaller version of an adult saddle may work but the wider flatter backed pony often with a good deal of Welsh or Arab blood is far more difficult.The tree needs to be fitted very carefully for this conformation - a flatter tree with flatter rails which is wide enough at the top of the arch to let out across the withers and with long enough points to get some purchase either side of the wither. Otherwise the saddle will sit too far off the back, and the saddle will slip forwards or to one side.
This is a "curlier" tree with a higher head and longer points: suitable for horses with a higher wither and more shape to their backs.
This is a "flatter" tree with a lower head and shorter points: suitable for horses with low withers and broard, flat backs.