Image size issue after recent 2015 Theme update

Since the recent TwentyFifteen theme update, I suddenly have an issue with the wrong image size being inserted into the visual editor when using the Classic Editor plug-in. Before the update, full size (2560×1920) images were automatically inserted from the media library, filling the entire column width. For some reason, medium size (300×225) images are now being inserted instead, filling less than half of the column width. This problem in the visual editor carries through to previews and the final product. Note: The “featured image” appearing at the top of each post is an exception, and does NOT exhibit this problem.

It’s possible to edit the images to be full size again after being inserted, but this is clumsy and time-consuming, and was never necessary before.

Tried deactivating all plug-ins other than Classic Editor, but that didn’t help. Switching browsers makes no difference. When Classic Editor is deactivated, image sizes appropriate for the size of the block selected are inserted using the Gutenberg (block) editor.

Site is on a shared server through Bluehost. Running TwentyFifteen theme, PHP version 7.3, and the following plug-ins in addition to Classic Editor: WP Edit, Really Simple SSL, Jetpack, Akismet Anti-Spam. All plug-ins and WordPress are fully updated. (There have been numerous updates of late.)

Seems like this issue might arise due to some default setting change within the TwentyFifteen theme? How do I fix this? Please help!