thumbnail gallery is resizing thumnails- why?

Hi I’m not sure if this is something that is happening since the last update or if it has always been this way, or if it is me.
The problem is this, when I put a thumbnail galley in a post it doesn’t keep them as thumbnails but enlarges them to take up any space available on the width of that line. Worst of all the quality then looks terrible.
All looks like I want it to in the visual editor, it is when you view the post itself. I am selecting the number of columns depending on the number of images. I have a screen shot of visual editor that I will put at the bottom of post itself as I don’t think I can add images here.
https://courses.loveyogaanatomy.com/gallery-test/
Thanks for your help I’m sure this must be a common problem.
Stu