I am using firebase storage for storing some files (maximum 30mb). My backend and frontend servers are deployed on Google Cloud Run.
Since last night, I am seeing this usage constantly after every 5min. Even without using the website my firebase storage has reached 1gb bytes stored.
I read that images stored in container registry use storage internally which causes the usage spike but I only have 1 image there, that too was deployed yesterday.
Why is it showing such a high usage after every 5min even if the website is not being used?

This problem is caused by google cloud run, because google cloud run access the website every 5 minutes. To keep it ready for incoming visitors. And you can learn more here.
To solve it you can follow these steps according to this documentation:
But keep in mind that there are quota limits which you can learn more about here. But I suggest making something like a verification system in your website since hackers can keep fetching your website and increasing the usage. I hope this helped