Image size

When I upload an image, for some images I can choose from four image sizes: Thumbnail, Medium, Large and Full size. But for some images I only get three choices, “Large” is missing.

Here is an example: https://gullregn.se/gullregn/folkhemmet/ For the image on the left you get four choices, but for the other two only three and cannot choose “Large”.

What is it due to?

I think I noticed that the page gets heavier to load if I choose “Full size” for images. What does the “Image size” setting mean for how heavy the page will be?

The page I need help with: [log in to see the link]