My favourite tool for this is http://grandperspectiv.sourceforge.net/ which gives a great graphical representation of files.

In your particular case, I have encountered issues with smaller "Solid State Drives" like those used in earlier MacBook Air system, where the system does not completely free up filespace on the SSD when some things have been deleted - perhaps waiting for some background tasks to take care of it eventually. To give the process a kickstart so to speak, I have run /Applications/Utilities/Disk Utility.app and used the "Erase Free Space" and that has freed up a lot of room that was not visibly being used when looking at any of the "file space" tools.

The older MacBookAir (2012 and before) machines do have have the SSD upgraded to something bigger. See for example: http://eshop.macsales.com/shop/SSD/OWC/Air-Retina

My favourite tool for this is http://grandperspectiv.sourceforge.net/ which gives a great graphical representation of files.

In your particular case, I have encountered issues with smaller "Solid State Drives" like those used in earlier MacBook Air system, where the system does not completely free up filespace on the SSD when some things have been deleted - perhaps waiting for some background tasks to take care of it eventually. To give the process a kickstart so to speak, I have run /Applications/Utilities/Disk Utility.app and used the "Erase Free Space" and that has freed up a lot of room that was not visibly being used when looking at any of the "file space" tools.

The older MacBookAir (2012 and before) machines can have the SSD upgraded to something bigger. See for example: http://eshop.macsales.com/shop/SSD/OWC/Air-Retina