In my newest wordpress website, I found something weird for the first time.
After setting up the new website and adding couple of pages, I decided to have the largest image larger.
Admin > Settings > Medias then change large image size to a bigger one ; 900 x 900 px > save
Went to upload new images (large ones over 1200 px) but once they are uploaded, it shows me that the largest size is around 625 px
PS I use WP Twenty Twelve theme
