Thanks Jeremy,
If I upload directly from Wordpress, I don't seem to have a problem. I tried that here: http://campwillowcreek.com/new-years-christmas-eve
But if I upload using a Lightroom export plugin called LR/blog, that's when I seem to run into problems. I uploaded the same pictures to this post: http://campwillowcreek.com/2013-12-31-test-with-lr-blog using that plugin and get the mismatched pics again. I will double-check that they are being exported at the same size, but I am pretty sure this Lightroom plugin is to blame.
Either way, Wordpress is working as it should and I appreciate all your help in narrowing down the issue and showing me how to fix the broken ones. Have a great weekend.