I see. These image sizes are controlled by WordPress. The determining factor of difference is the aspect ratio of the images you uploaded. The ones you uploaded have an aspect ratio of 1:1, whereas the ones in the demo are 3:2, thus the difference.
Would you like to resize your images to a similar aspect ratio through code? Doing this may crop the image though. If that’s alright with you, you can try adding this code through Appearance > Customize > Additional CSS:
.single .featured-image img, .blog .post-image img {
aspect-ratio: 3/2;
object-fit: cover;
object-position: top center;
}