One of my friend's iPad was running out of space, there were not much pictures and music. I was suspecting a movie that's causing the storage/space issue, but I couldn't find any big file to delete, I've been to TV, Music, iTunes purchased but it's not adding up the total storage.

I downloaded the free trial of AnyTrans by iMobie -

Plugin the iPad to my computer, Trust the computer via the iPad screen, then went to File System -> System -> Purchases -> then from there I was able to view two movies with size over 2.4GB.

Using the free trial, I was able to download the movie to my computer before deleting it completely.

The disadvantage is as you can see, movie has random name so you will have to rename it later after you've downloaded it to your computer.

With the iPad plugin to your computer, go to your iPad Settings -> General and look at your storage and you will be find free more storage.