Low On Diskspace error

Hello there, today I just encountered this problem for the very first time, I tried to download this video from drive, it was around 2.8 GB. Meanwhile the iPhone has at least 15 GB available, I nuked most of the stuff I don’t need before already, but now I don’t know why it show that notification about low on disk space. Truly making me confused, please leave an answer if you could. Thank you.