I know it’s been a while on this, but I figured out the issue while trying to fix it myself. When using the portfolio grid it links to the ‘large size’ set by WordPress. To change this go to settings>media and change the max width and height of your Large Size. Once this is complete you’ll need to regenerate your thumbnails. I use the plugin “Regenerate Thumbnails”.
Yes I did. Bummer. I’ll just replace the slider with something else. Thanks for the response.