Adding pie chart via external axes method

Hello,

I have been using `returnfig=True` and adding a couple of things to customise my chart, but when adding a pie chart I recieve the follow error:

> ValueError: Image size of 65243x662049 pixels is too large. It must be less than 2^16 in each direction.

I have tried reducing everything so that the pie chart can be as bareboned and as rudimentary as possible, but the result remains the same. I have been using the pie chart successfully in standard matplotlib so I feeling like it should be working and I am at a bit of a loss.

Start by reproducing the error with `returnfig=True`, an external axes, and a pie chart, then compare that setup with the working standard Matplotlib case. The issue is resolved when adding the pie chart no longer produces an oversized image error.

Written by the indexing model from the issue text.