Adjust Featured Image Height

It’s possible. The proper way would be a bit complicated. You can look at functions.php to see how they’re added.

The other way is using CSS to make the image appear in a different height.
Example:

For blog/archive/home page
.blog-view .entry-thumbnail img {height:100px;}

For single pages
.single-view .featured-image img {height:100px;}

Though I’m wondering what complaints you’re getting as your images look fine from my end.

You can also just disable them here
Frontier Options -> Blog View -> Thumbnails
Frontier Options -> Single View -> Featured Image