If you open Mapster (I have the window resized) and you right click on the map to show the rest of the continent, if you try and mouse over another zone it doesn't highlight the zone you're over. I've tried resizing the map, etc. and I can't find a way to get the coordinates to match up.

Mapster by itself will try to compensate for size changes. Other addons have a history of breaking this. Unfortunately there is nothing I can do to prevent other addons from interfering.

1.10.1 doesn't seems to have that problem tho, just had to revert after the latest to 1.10.2 update.

size/scale change had to be turned off because it was causing bad UI issues. There was one thing left in 10.2 that caused an issue, should hopefully be resolved in 1.10.3 - although size change is of course still disabled at this time.