I have done some testing, and apparently it's because of the post titled Hippopotomonstrosesquipedaliophobia
When someone comments on the post, it brings it up in the Recent Comments widget, and the long title essentially "pushes" the Gravatar over, making it appear smaller and smaller, depending on the length of the title.
There are a few solutions/options:
* You can ignore it best you can (less than optimal)
* You can use smaller titles (I don't recommend this - it's too restrictive!)
* You can switch themes to something with a larger sidebar
* You can purchase the Custom Design upgrade and add the CSS to make that sidebar a bit wider to accommodate these longer titles and the Gravatars simultaneously (I personally like this option the best for you getting what you want looking just right, but the downside is that there is a cost involved)
Let me know if you'd like more information or you'd like help getting anything set up.
I'm very sorry for the inconvenience!