It was the child theme. My edits to footer.php added two things, but those edits were on a base of footer.php as of summer 2020 (v4?). I started over with the v5 footer.php and added my two customizations and the Featured Image size anomaly cleared up.
There remained two formatting changes that v5 caused:
— The main menu had been flush-left, with v5 it became centered. Fixed in Weaver Xtreme admin > Main Options > Menus > Primary Menu Bar.
— The Search box changed line height (smaller) and lost its background color. Tried re-establishing the background color in admin > Main Options > Content Areas > Search Boxes, but it did not fix it. There is no control there for line height but I can probably run down the affected ID or class and apply that.
Thanks for your help.