add_image_size limiting sizes (???)

WordPress has its own built-in image sizes. These sizes are:

thumbnail    // Thumbnail (default 150px x 150px max)
medium       // Medium resolution (default 300px x 300px max)
medium_large // Medium-large resolution (default 768px x null max)
large        // Large resolution (default 640px x 640px max)

The medium-large image size has no height set, meaning that it will only crop the image by width.

You should also regenerate the thumbnails after changing the thumbnail sizes. That might be why you are getting incorrect sizes. You can use the Regenerate Thumbnails to do so.