At first I thought it was the website being wacky.
But then I Inspected your Report button (originally a little 16x16px flag) and your icon is huge bc it's displaying more or less 200x200px.
Is this report button round thing custom? Is it in your layout? Can you find the line of code this icon is in?
The help might be something like:
<img src="YOUR_CUSTOM_IMAGE_URL" width="16" height="16" alt="Report">
<img src="YOUR_CUSTOM_IMAGE_URL" style="width: 16px; height: 16px;" alt="Report">
> are the < and > brackets btw
EDIT:
TRY CHANGING THE MAXIMUM SIZE OF THE ICON (easiest route, CSS):
.cb-content-cell img, .comment-reply img {max-height: 36px !important;}