I read "enlarging the map" to mean either: 1) increasing the map magnification (zooming in more), or 2) expanding to a wider area (e.g., a map of England expands to a map of Europe and which expands further to Europe and Central Asia).
You most probably meant no. 1, right? If that is the case, I suggest:
"Click + to increase map magnification" is suffice. This instruction is always valid because every click will increase whatever that is currently being displayed.
Or perhaps:
"Map magnification increases with every subsequent/successive click of +"
Chris