We have updated the way WordPress.com resizes your images. The update is designed to allow images to resize properly after a theme switch and it will also make it so sizes set in Media > Settings are applied to images using classes such as "size-medium" (common).
If this update changed your images in a way you didn't expect, please see
this reply where we've started looking into things with you:
We will continue to monitor the issue, and we may make additional updates based on your feedback.