How to solve this problem
Step 1) generate report
1. Sign in to ArcGIS Online with an Administrator role. And click on the Organization tab.
2. Go to the Status blue tab.
3. Click on Reports.
4. Click on Create report.
Once you are there a window pop-up :
5. Choose item report type.
6. Then you can schedule reports or not, and Click on Create report.
You will get a .csv file and you can filter it by Feature Storage Size (bytes), it will give you the member of your organization who created the file, date, and some other details.
Ask your org's administrator to share a screenshot of the following view: