were the Aztecs successful at building a huge empire because they were smart, strong or a little of both?

A little of both

Explanation:

They were smart because they had impressive architecture and had great agriculture, irrigation, a complex calender system, hieroglifics and etc. They had to be somewhat strong for certain things such as agriculture and architecture building which required labor, and while they had laborers to do that they were still Aztecs. So both because of they're architecture and agriculture and strong because those things requieres muscle.