A few things I can think of:
One solution might be to go the donut chart sheet, go to the marks card and click on Size. then slide the size way up. This will increase your pie chart size, and thus making the ratio between donut chart and margin more to your liking.
The other solution is to right click on the margin, and click format sheet. Click on the paint bucket that appears in the formatting box on the left, and change your worksheet shading to be 'None' (this will get rid of the margin completely)
So, the size trick helps a ton. The no shade trick just gives it a white background
What if i just gave it a white background and put my donuts in the circles? like this
I don't entirely understand your question. Could you rephrase it? Does what look stupid?