I have my SSD partitioned into two 128 GB disks. I was managing my disk space and decided to shrink one of my partitions (the one with Mavericks) because it had more free disk space, in order to transfer this space into my main drive (the one with El Capitan).
As I couldn't just transfer space from Mavericks to El Capitan I shrank my Mavericks partition, but then my total disk space was reduced. I did everything I could without success.
Macbook Pro Retina Late 2013 13" (256 GB SSD)