Images smaller in posts (large in media)

Hi, in posts, I like to insert images. I upload photos to Media first. The sizes seem fine and are original. When put into posts, the size 550px images from media for some reason reduce to around 300px - how can i keep the image sizes in posts?

I've tried using the stretching tools within the posts, but it seems the quality is reduced and this just produces large pixel poor quality images?

The do seem to be set onto 100% in the posts too.

i've gone into media settings, and tried setting medium to 1000px, but this doesn't make a difference.

Thanks for any ideas!