Using on-the-fly image resizing services would be a way forward in this scenario
Google has an undocumented endpoint used for on-the-fly image resizing. As explained in https://czm.io/posts/2013/04/google-image-resizer/, that endpoint is as follows -
Base URL
https://images1-focus-opensocial.googleusercontent.com/gadgets/proxy
Parameters:
url: original image URL
container: must be “focus”
refresh: time (in seconds) to cache it on G’s servers
resize_w: width in pixels
resize_h: height in pixels You can either specify both
resize_* parameters or just one.
For making sure that you only resize external images, you will need to set up a Blogger conditional for testing if the thumbnail exists or not (data:post.thumbnailUrl is always non-existent for external images excluding post containing Youtube videos)
As Google's endpoint is undocumented, we can't be sure about its availability in future (Due to the fact that this is related to Google+ social network which is itself closing down in April 2019). There are multiple alternatives for on-the-fly image resizing services which are both paid (like https://kraken.io/docs/image-resizing, https://www.imgix.com/) and self-hosted (like https://github.com/jimmynicol/image-resizer)