Why is my macbook pro (late 2013) running so much hotter with the Nvidia gpu enabled?

I know there are plenty of threads about overheating MacBooks, but this is not one of them. I have one of these temperature/fan apps installed and running, and only very rarely I'm over 70 degrees for more than a minute. However, since my Macbook is really old now, I thought about removing the 'Automatics graphics switching' option, meaning to activate permanently the Nvidia GPU, instead of the graphics chip on the processor (and maybe have a better performing Mac that way). But I really don't notice a difference in performance, however it makes the computer run hotter very noticeably, at least 10 degrees, even up to 20 degrees (Celsius). What is the reason for that? I understand that activating a GPU will produce heat, but at the same the integrated graphics chip is deactivated and *should* produce less heat, and the computer doesn't have to work any harder, so why should it get hotter? After all, a more powerful chip (like the Nvidia) shouldn't work very hard just to watch videos, so why does it get so hot?