Whether you're thin or overweight, it all depends on your fitness level. I went (not with Disney, but another tour company) and our group included people of all shapes and sizes, including one lady with a cane, and everyone was able to enjoy the tour. There are several different "paths" to take at Macchu Picchu, and they don't all involve walking up mountains. In the case of my tour, and I'm guessing Disney will be similar, we were frequently offered options - take the harder hike with one guide or take the easier way with another guide. I think you'll be fine.