Imovie taking up space on my iphone

I use Imovie to take video clips. Once the clips are uploaded to onedrive I delete them so there is never more than about a dozen 5 minute clips on my phone at a time. Once I delete the clips I also delete them from my photo library and Icloud. Imovie is currently taking up 16.5 gb on my phone. I don't understand how this is possible! How do I clear this? Almost all the articles I found have to do with the Mac. I found an old article about clearing the cache but it's an old version of the app and the info is no longer accurate. Thanks for the advice.