The first is that a vast Empire can muster far more in resources than a region as small as Portugal or even England. In the west, none of them could martial anything like the resources available to Rome and hence could not do many of the things that Rome could do. The possible exception was Carolingian France, which, while not on the same scale as Rome, was able to martial very significant resources. There are many reasons and no single one was decisive. My favorite is cannons.
Cannons make up for a lot.
By the way, I don't think that the Chinese were techologically superior by 1500. Europe had made great strides and, in my opinion, caught up in many areas by then. But of course, opinions vary.