With both pie charts and donut charts, the size of a segment is based on its arc length. With a pie, that's the same thing as area, but for donuts it's not. So I guess that's one argument against donut charts. Neither chart is great for comparing sizes imo.
A donut chart is also area<->arc length proportional. Intuition: a donut chart is what remains of a pie chart when a smaller version of the same pie chart is removed from the center. If the two pie charts are proportionally correct, their difference must be proportionally correct.