IMovie IOS Storage full

I have a 256GB Iphone XS that according to settings, is only 74GB full. But I cannot save or export a 4K or 1080p movie from IMovie as it just displays the message "There is not enough space left on this device"?

The movie clip is just under 1 hour long and it's estimated to be about 40GB. So any ideas why I can't download this to my iphone?

I have tried other apps such as Splice, and the final export also fails.

Many thanks in advance!