Hello

I am also working on an editor My saved map with dimensions 100x100 and 100 objects on it has ~37 kb in size. I am using binary files to store data. They are fast and easy, have you tried them? They are similar in usage to the streams showed by Legolas. Also to achieve a small map in size is to store in it only the things which you really need.
For example if you would like to save to your map 100 Objects which are different only in the terms of position, do not save all their properties. Save an ID which represents such object and its position.