> ***TL;DR***: I disagree with the OP, and believe the current UI works for this site (that is, the Green Answer Accepted checkbox circle is **fine**); therefore, we do not need to be having everything aligning perfectly to a set size, nor do we need to even be discussing this issue further...  
> <sub>(This last part is obviously my own opinion, regarding this discussion not needing to go further)</sub>

I disagree with you, and believe there is nothing wrong.

Here's a zoomed in snippet, plus two black lines to show in-line size of the checkmark itself:

[![SNIPPET!][1]][1]

Here's what I see:

 1. The check box itself is slightly off-center from the rest of the buttons - that is, the checkmark *itself*.
 2. The green circle for an "Accepted" answer is slightly outside the size of these black lines... but ***only just so***.  I don't see an issue with this, namely because the green circle has spacing between its edge and the checkmark itself to not look crazy weird.
 3. The green circle is there ***for emphasis***.  It's done this way so people KNOW what question is answered.

This issue you've stated would only really apply if it were a case of the entire checkmark image itself being too huge - the checkmark itself is fine, the emphasis circle is slightly larger, but not such that it impacts UI.  Now, if this were on the [Science Fiction and Fantasy](http://scifi.stackexchange.com) site, then this would be an issue if the checkbox image itself were this way, and because I'm insane, here's a snippet from there:

[![enter image description here][2]][2]

The SciFi page uses circular buttons anyways - which means it must flow in line there (the black lines are my own).  Both of these are zoomed in.  But if we zoomed out, we'd see a case of our green circle not really going far outside the lines anyways - thereby not causing a substantial UI impact.

My belief is the green circle ***fits fine into the current UI of Ask Ubuntu***.  We do not need to change it, and the slightly-larger-than-the-vote-buttons green answer-accepted circle is fine.

  [1]: https://i.sstatic.net/mA8RT.png
  [2]: https://i.sstatic.net/x7d8L.png