Youre really looking at the wrong way. 😊
The SSD of the Air and macbook Pro Retina SSD are "working platform" storage.
Your best practical option is buying a 1TB external HD for $65
128gig is PLENTY enough......for all your APPS and working files. Buy as much as you can reasonably afford, just remember that LARGE files are best kept off the macbook and on external storage, which is far cheaper.
You, regardless of internal HD or SSD, need an external backup for time machine and same for important file storage.
The "low" capacity of the SSD on the Air and entry level Retina are meant to be working platforms for these computers.
Meaning ALL your programs and your working data (stuff you use at least every week).
To try to use either the Air or Retina as a massive storage platform of any kind (lots of pics, music, movies etc.) is both not indicated or practical.
external storage, quality too, is absurdly cheap right now. 😊
superslim external HD.
see this: only 7mm thick
http://www.amazon.com/Toshiba-Canvio-Portable-External-Drive/dp/B009F1CXI2/ref=s r_1_1?s=electronics&ie=UTF8&qid=1377642728&sr=1-1&keywords=toshiba+slim+500gb
Or better still this HUGE HD, which is very small, very reliable and very cheap:
2 Terabytes, 2.5" and only $119
http://www.amazon.com/Toshiba-Canvio-Basics-Portable-HDTB120XK3CA/dp/B00ARJD56K
Or for $7 more is updated "design" casing cousin for $127
http://www.amazon.com/Toshiba-Canvio-Connect-Portable-HDTC720XK3C1/dp/B00CGUMS48 /ref=sr_1_4?s=electronics&ie=UTF8&qid=1379182740&sr=1-4&keywords=2tb+toshiba
