Really short one this - after the discovery of the Americas the campaign minimap is enlarged to the left to allow for the extra landmass.

This is done with a script command in the campaign_script file.

restrict_strat_radar false
Which changes this:

to this:

And then:

restrict_strat_radar true
changes it back again.

Not sure how much use we can get out of this one, whether when we get access to the graphic files if we can mod the area it covers - but that's how it works at least.