Image size keeps changing

I’m finding that the calculated size of the fancybox images, whilst mostly consistent, occasionally changes as the forward and back arrows are pressed. In other words, the same image will appear larger or smaller (seemingly at random) as the gallery is navigated. Why does this happen and is there a way to keep the image size constant, either by a CSS override or some custom javascript options? See link for example. The issue, although relatively subtle, is driving my client crazy.

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