From @mattwoodrow from investigating https://bugs.webkit.org/show_bug.cgi?id=272186:

Chrome seems to be setting the size of their ::view-transition-group element to that of the snapshot containing block. That seems wrong, but visually what you’d expect to happen.

Capture the image algorithm has a special case for sizing the image when capturing the root, I suspect Update pseudo-element styles should have the equivalent wording for setting the width/height style properties.