The problem with smaller headed horses is WHERE they are smaller headed. Over the years, I have fitted hundreds and hundreds of bridles – as have many of us. There are reasons for Arabian sized, cob sized and warmblood sized: they each tend to be “larger” or “smaller” in particular spots.
If you are looking at a cob size bridle, for instance, they tend to be shorter in the cheeks, but longer in the browband, and have a larger (and proportionately thicker) cavesson. If your horse is slender of face, the browband may be too long, and the cavesson may look too clunky.
Best advice would be to call Dover, or some of the other good quality saddlerys, and see how they measure their bridles. Measure your horse’s head the same way. Give the saddlery the measurements and have a bridle put together, or made.
Long ago, bridles were nearly all custom made! There were no buckles and the bits were stitched in. Nowadays, bridles are meant to fit a broader variety of horses. A good bridle will last decades – I have a couple that are 40 years old! – if well cared for. Makes the extra money paid for a good bridle well worth the cost.