Carousel: Any way to make thumbnails smaller?

Tiled Galleries are built to match your theme’s $content_width variable, as defined in the functions.php file. You can read more about it here:
https://codex.www.ads-software.com/Content_Width

As long as this variable will exist, your galleries will extend to match that value, regardless of the number of images in the gallery.

You could change the size of this particular image with CSS, by adding the following CSS to your theme stylesheet or to your custom CSS editor, available under Appearance > Edit CSS in your dashboard:

.page-id-15 .tiled-gallery.type-circle .tiled-gallery-item img {
    width: 200px !important;
    height: 200px !important;
}

I hope this helps.