Everyone who has ever tried to watch a full movie on an iPhone will probably agree that it's not the best way to see the latest blockbuster. On the other hand, a smartphone works just fine when you're watching a funny YouTube clip while commuting home from work.
The point is: different devices are suited for different types of video content. A study by Ooyala confirms this: while only 49% of the time spent watching video on smartphones is spent with videos longer than 10 minutes, long-form video accounts for more than 80% of total video consumption on connected TVs.
Interestingly, the rule is not as simple as "the larger the screen the longer the content." Tablets such as the iPad, which typically have smaller screens than desktop PCs such at the iMac, see a higher percentage of long-form video than PCs.