Are YouTube file sizes smaller on iPhone?

I have a data limit on my phone, and my Wi-Fi. My question is, if I were to watch the same youtube video on my computer vs my iPhone, would the ammount of data used be the same on both devices, or is the file size smaller when watched on an iPhone?

Extra credit: about how big is the file size on an hour long YouTube video on an iPhone?

Would be the same. The size depends on the "change quality" icon in the bottom bar of the video, not your viewing device.

average file size at medium quality at 1 hour with youtube's compression is about 300mb, twice as much for high quality