Pretty much exactly what Blob said, for me. It's all about what you want out of your device and what you're using it for. For me, like Blob said, if I don't have to sacrifice performance or risk hardware damage (be it heating, it being so ridiculously thin that it's actually at risk to break, etc.) then of course I'd want it thinner simply because if the device is portable...well, more portability is better save for the aforementioned downgrades/risks. I'm definitely not going to give a shit if my laptop/iPhone/iPad is thinner if the overall performance is downgraded, but again, that's me. Maybe someone needs it for very minimal use and has it out constantly, I'm guessing it'd be perfect for that person.
So yeah, size absolutely matters but that doesn't mean anything in regards to how thin it is (or if you just really like lugging around a huge laptop, how big..?). For me, the iPhone 6+ is perfect because it's big enough that I can watch movies and such on it, as I do very often, but small enough that it still fits in my pocket and I can use it one-handed (because apparently I'm a fucking caveman). For others, it's ridiculously too big. Alternatively I also wouldn't want the phone any thinner because I personally like the weight of the phone and wouldn't want feeling that it's going to snap.