Big legs = Big upper body?!

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I've also heard that doing heavy leg work will increase your testosterone.
I think one of the keys is that heavy leg exercises also tend to build the rest of your body. Squats, for example, really work the quadriceps, hamstrings and glutes, but they also work your back muscles and core. Deadlifts hit the hamstrings especially hard, but they also build your upper back, shoulders, and forearms. So people that do a lot of squats and deadlifts are actually hitting their upper body pretty hard, too. While it's possible to build your upper body without ever getting your legs, it's difficult to build your legs without involving the upper body, too.
Testosterone by itself won't build muscle. It just allows you to work harder and get better results. If there is a hormonal effect of leg work, it's wasted on the upper body unless you do some upper body work. I mean, you can inject all the testorone you want but if you never work out you won't get bigger muscles.