In reply to Suhaib Khan.

Try the Regenerate Thumbnails Plugin.

Each Theme defines the width and height of the featured image, and WordPress creates images with exactly the sizes specified. So when you switch from one Theme to another, all of your featured images have already been generated according to the size(s) defined by the previous Theme. So, when the new Theme attempts to display a featured image, WordPress doesn't have one at the size defined by the new Theme, and attempts to display the next-closest size for that image.

By using a Plugin such as Regenerate Thumbnails when you switch Theme, you can force WordPress to resize your featured images.

Note: this is the sort of question that is ideally suited for the WordPress support forums. There are lots of people there who can help with issues such as this one, and I'm sure this specific question has been asked and answered many times.