There is no any bug in the feature slider you can see that in the demo too https://demo.acmethemes.com/supermagpro/ , If you inspect you slider image in your site, the all slider images are 300 * 300 in size. Please see this image link http://prnt.sc/atsxms .
This usually happened because of images size here Go to Settings > Media. This is not related to actual image size you upload.
If you cannot do it in your site, please provide us wp-admin details, we will see your issue.