Sound like limitation between 32 and 64 bit system. Or the driver is limited in a way.
You should be able to reverse that to original size using partition software.
I don't know mac a lot it could be something the OS does.
Maybe there's unformatted space on that SSD. Go to Apple Menu and select "About This Mac". Then choose "System Report". Under "Hardware", click on "Storage". This will give you info on which SSD you have and its capacity.