Its because its too large , if you could stretch the window it would look better! I would guess the opera user has a larger screen?
You need to go buy a bigger monitor! Seriously though, it will be addressed when we have some user preferences in place; one of them will allow you to set the size of the "large" map view.
Unfortunately if on full screen view its still breaking up then you'll have to do without large map view until we have user preferences sorted out.
Or you get a higher screen resolution
The 19 squares you show are for a screen resolution of 1024 across the top. This WILL be catered for when we have user preferences but for now Large Map View needs at least 1280 pixels.