It depends on the the streaming service you are using and the quality setting in the app/device. For Netflix for example its:
Echoing what @ YetAnotherAcc has mentioned, streaming can vary from the type of movie and where this was watched/streamed from. Quality sounds like it is a big factor here as well judging from the size of movies.
I should also mention that if this was on WiFi of any nature, there may possibly be other external usages (Updates, Messages, Notifications) that could contribute to the total 8GB used here