Images imported, but different sizes not generated in target site

I am importing a 1500+ posts and about 900 images from one WordPress site into another (which already has about 500 posts of its own). Both sites are running the most current version of WordPress. When I export the content from the source site, the images in the post content look like this in the XML file:
[source url]/wp-content/uploads/2018/09/aspens-300x206.jpg
And this works fine in the target site. But when I run the import, only the original media file itself is imported, and no other sizes are generated. Thus in the target site, the imported src is the same (aspens-300×206.jpg), but that smaller version of the image is not generated on or moved over to the target site. The image exists in the media library there as only aspens.jpg.