When it all comes down to it, any exercise that allows you to place consistent tension on the targeted muscle and that allows for a progressive increase in resistance over time will result in muscular hypertrophy. The leg extension allows you to accomplish both of these things, and so yes, it certainly will add size to your quads over time.

So, leg extensions can be a valuable addition to your fitness routine if you're looking to fortify your knees for your favourite sport, prevent future injuries, or recover from a knee issue.