The recipe you posted is prepared in a slow cooker. That might be part of the problem. As it cooks, any juice or fat that is released from the meat will contribute to the overall liquid in the dish, rather than evaporating as it would in a stove top skillet/pan.
Typically when I make a curry and the consistency is thin, I'll remove the cover and let it reduce a bit. That isn't an option with a slow cooker because that isn't the way they are designed to work.
For the specific recipe you posted, I would suggest using a leaner cut of meat. You might also want to drain the tomatoes, as well as use Greek style yogurt, which has a thicker consistency.
Or you could always eliminate the slow cooker and try a more traditionally prepared recipe. Perhaps something like this one: