post_thumbnail is not a thumbnail

The post_thumbnail name is indeed a bit confusing. When Featured Images were introduced in WordPress, they were originally called “Post Thumbnails”. The name was changed to “Featured Images” a release later, but the code still refers to Post Thumbnails everywhere. Here are a few examples:
https://codex.www.ads-software.com/Post_Thumbnails
https://developer.www.ads-software.com/reference/functions/the_post_thumbnail/
https://developer.www.ads-software.com/reference/functions/get_the_post_thumbnail/

If you’d like to get different image sizes for that Featured Image, you can use the media ID from that post_thumbnail field, and use the Media endpoint to get more info about that media element:
https://developer.wordpress.com/docs/api/1.1/get/sites/%24site/media/%24media_ID/

You’ll get a thumbnails field in the response, with the different sizes available to you.

I hope this helps.

Thanks.

I want to get the post and the thumbnail in one call. I can do this with the “JSON API” plugin, can I do it with Jetpack?

I’m afraid that won’t be possible. Like with the WP REST API, the /posts/%post_id%/ call doesn’t return exact info about a Featured Image.

Instead, if you know the size of your resized thumbnails, I’d suggest that you create the thumbnail URL manually, by adding -150x150 to your Featured Image URL.