I've created a child theme for twentythirteen and in it I've added an index file and a category file which are 'full width'. The code for each is identical, yet index.php pulls the full-sized featured image above the title, whilst category.php pulls the featured image sized according to the set_post_thumbnail_size set in functions.php. I have deactivated all plugins.
I thought it might have something to do with 'loop','index', which I found in another thread, but I've searched the entire folder and can't find any reference to that.
What is causing the category.php to obey the featured image size and the index.php to ignore it? As it happens I would like the category.php to do the same, ignore the thumbnail size.