I see that the issue is related to the bundle size of your application. Although your project code maybe small but after installing the dependencies it might be exceeding the 250 MB limit.
Good news — Vercel has increased the Python Functions bundle size limit to 500MB unzipped (up from 250MB).
statsmodels is one of those packages that’s deceptively large once all its dependencies are unpacked. With the new limit, your Flask project should deploy without any changes to your requirements.txt. Give it a redeploy and let us know!