We see a lot of polo saddles, and we see a lot of ill-fitting polo saddles. Some times the saddle being used is just not a good fit for a specific horse, but quite often, the saddle used to fit but doesn't any more.
Horses do change shape, with age, with fitness, or as the result of accidents or disease. But, a persistent issue is that many polo saddles themselves change shape. - they spread, i.e. they get wider at the shoulders (and lower at the withers). Generally, spreading is caused by the long term application of downward force on the head plate, or gullet plate, which reinforce the head of the saddle tree. In polo in particular, the downward force can be several times the player's weight, and this combined by the variation in force, is more than the head plates used by many polo saddle makers can withstand.
The problem of spreading is so pervasive in ;polo, that you would think that saddle makers had cottoned on to this and come up with appropriate solutions, but with a few exceptions, it is still not the case, and the cheaper the saddles get, the less likely it is that the issue will be addressed comprehensively.
The saddle on the left ( a well known brand) used to have the same shape