The length of the video
shouldn't make a difference, the resolution does as does the type of the connection - Safari appears to buffer more on certain types of connection or if it determines that the connection is poor. This isn't documented but is observable. AFAIK there is no such API for getting this information in iOS, so it's doing something arbitrary or using a close function taking I/O across the platform as a whole. But back to the point, I'd like think that the PS4 would likewise be smart about buffering streams, depending on what else is going on, e.g. HDD activity, network activity.
Also, I'm not using the iOS YouTube app. Most of the videos I watch are those on the webpages of sites like EuroGamer or C&VG, which appear inline on the webpage. I'm not using the Vita for YouTube and I didn't bring it up, why do people keep mentioning this?