In R2014b and later, if you save a graphics object in a MAT-file, MATLAB stores all the data for that object. When you load the MAT-file, MATLAB regenerates the objects. If the graphics object contains a lot of data, then the file might be large. Saving or loading large files can be slow.
In previous releases, if you save a graphics object to a MAT-file, MATLAB stores the object as a double. Storing a double does not require a large file. However, MATLAB cannot regenerate the object when you load the file.