Ok, I can see 2 reasons why your thumbnail is too small.
1. When you select 'add media', choose the image you want and then on the right you need to check 2 menus. In the first one (attachment menu), you can change an option of the way the image appears on your site. If you want to have the effect as on the site kimkermode.com, you should select a 'media file' (the first in the 'attachment menu'). Then choose dimensions of your thumbnail from the second menu (this menu is below the first one). It should be the dimension you want to appear.
2. Ensure the media files (images) are preapared for the site. To have the same dimensions on the site, all the files should be the same width. For example, the pics on kimkermode.com are 626px width. Both! That's quite good dimension for the pics for your site.
When you got what I wrote above, it should work exactly as you want.
p.s. I work on a Polish language version of WordPress so the names of menus and all the buttons I wrote about could be slightly different cause this is my translation (thats why I explained the position of those buttons;)