Manufacturers seem to be doing impressive things in small packages now (the razer blade comes to mind), but even so a GTX 1060 can't boost as high in small packages due to the thermals. Higher heat also decreases reliability/lifespan. Also, opening up some of these rigs for cleaning can be fairly annoying/difficult if you ever want to repaste, or clean fans (RAM and drives are usually accessible without much trouble)
If I wanted a 15 inch laptop I would first ask myself how much horsepower I need, then find some benchmarks. If that size laptop can do what I want with decent thermals then great, but if more power is required then I'd move up to a 17 inch model.
Also, the screen size does make a difference. I feel more absorbed by a 17 inch laptop than a 15 inch one simply from the display size.