There might be something to that, as my CPM on my main channel (where most of my content is between 3-6 minutes) is around 7 bucks, and the CPM on my second channel (where most of my content is between 6-10 minutes) is around 13$.
The problem is though that other factors come into play as well. My main channel with the lower CPM has a predominantly older male audience that's mostly based in the USA - plus my retention rate is low. Around 25%.
My secondary channel with the higher CPM has a much larger global audience that spans every age group. It has a LOT more interaction (comments, likes, favorites), and it's retention rate is through the roof (around 75%).
I feel like these reasons pull the CPM the most, but length of the videos themselves may be a factor!