Reported by Erik Zachte on MediaWiki.org:
[After clicking the full screen icon] the image is then downsized to a small thumbnail and stays that way even after clicking same icon again to leave full screen mode. I use Windows 7, Chrome 33.0.1750.117, screen resolution 2560x1600
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Seconded with this browser information:
Mac OS 10.9, Chrome Version 33.0.1750.117, Retina Display
Version: unspecified
Severity: normal