It's just a minor CSS issue. See the following gif:
This has been resolved as part of design overhaul MSE went through not long ago. Now both normal and hovered states have blue square background of the same size, as it should be.
Looks like the problem is that there class
Simply removing the
Ps. I'm pretty sure this also affects several other SE sites besides MSE, including (at least) the recently graduated Academia.SE and graphicdesign.SE.
Actually, the real cause of this bug seems to be that the bounty icon styling on this site (and on Academia / Graphic Design / etc.) is a Frankenstein hybrid of two entirely different styles:
SOFU style (SO):
Beta style (crypto.SE):
Frankenstein style (meta.SE):
Frankly, I suspect that whoever designed these stylesheets simply started from the beta styles and tweaked them to look more like SO, but forgot to modify or test the unawarded bounty appearance.