My file system claims to only have 4.5 GB left. While my OS (a folder with in file system) still has 75.2 GB left. I put something near 130 GB on my Ubuntu partition, it should have enough space. I confirmed that I can put things in OS that exceed the space in available file systems, but that makes no sense, OS is listed as a folder inside of file system, why would it have more space than it's parent folder? What is going on?

Here is the output of df:

Filesystem 1K-blocks Used Available Use% Mounted on 
/dev/sda5 113773200 103741440 4252408 97% / 
udev 2004600 4 2004596 1% /dev 
tmpfs 804756 848 803908 1% /run 
none 5120 0 5120 0% /run/lock 
none 2011884 436 2011448 1% /run/shm 
/dev/sda2 127526908 54045584 73481324 43% /media/OS 
/dev/sda3 39144708 89016 39055692 1% /media/DATA`