14GB is almost 16gB. Your G: drive may be internal to the phone, not the SD card. (Memory manufcturers consider a billion bytes to be 1,000 * 1,000 * 1,000. Computers consider 1 billion to be 1,024 * 1,024 * 1,024. So the computer reports a smaller number, because it's reporting a different measurement. It's like a US gallon vs. an Imperial gallon. They're both "gallons", but they're different amounts.)
But ... if you want to make one large partition on the card, get a card reader, so you can plug the card directly into the computer and try Device Manager to delete all partitions on the card, then create one partition using all available space. Or you can download GPartEd, burn it to a CD, boot your computer with that and use it to repartition and format the card. (Format it as FAT32.) If you partition it with Device Manager, you'll have to then right-click the carc and choose Format. (Again - FAT32.)