if your subject tree bears plenty, you can try thinning the fruitlets this summer and see if the remaining fruits will improve in size. With jujus, there is also the likelihood that the variety you have may be true to name, and the ones you see online(or vice-versa) may have been mislabeled by the farmhand and broadcast to buyers, who then share and pass along budwood. Totally unwittingly, even by supposedly reputable nurseries.
photos of fruits in a totally different climate also lie sometimes, especially if one is using the relative size of the foliage as basis for fruit-size approximation. Trees grown in full sun, hot weather, and with longer growing seasons generally have smaller foliage. And having mentioned that, it may also be due to the overall photosynthetic activity per unit time which equates to bigger fruits. That field station in new mexico probably gets more sunny days and longer daylight hours overall.