Can you provide your sample workbook?
Please share sample workbook/data.
I generally don't see the lat/long axes in your picture, so I'm guessing you're using a shape file? If so, what kind of shape file are you using?
I actually recognized several countries (Australia, New Zealand, and Papua New Guinea to name a few)-- they are in their respective positions, but they are a mere fraction of their correct size. (Drop the Country info on Tooltip and see!)
And while I don't work much with maps, this makes me wonder if your shape file has somehow become corrupted OR it's simply not communicating using the correct "units".
#1 Go into your Map layers and RESET... see if that helps out.
#2 Try switching to the Tableau default map (if you're not already using it).
#3 Are you able to modify the Size widget? This usually has no effect with maps, but I thought I'd suggest it.
#4 Confirm that your shape file measurements and map units are the same.
Good luck to you!
Glad you were able to find a resolution! Please see my suggestion #2 above