I don't have the idea if it's EXT2/3 file system that makes Ubuntu think that my Ubuntu installation has only 0 bytes remaining space -- but I suspect that it is so.
Asking other average Ubuntu users, they also don't know where the problem really lies. It's probably because they natively installed Ubuntu on large partition -- by the way, this 0 bytes problem are commonly experienced by WUBI users.
But they know the solution: "delete unnecessary files, uninstall unused applications"
And so I did. I deleted about 100MB files yet, it's still saying 0 bytes.
I removed application that I don't usually use and I freed about 350MB of space. And yet, it's still saying 0 bytes.
So where does the problem lie? I really suspect it's how EXT3 work. And after reading an article that "Why doesn't [GNU/]Linux need defragmenting", it's all clear.
For Ubuntu not to be defragmented, it should at least have 5% free space to work on it's non-defragmenting trick. Unlike NTFS that it let's you consume to the last remaining free space -- I think. :D
So what exactly is the solution here? There's actually just two way to repair it.
a. Resize your partition.
b. Delete more files, uninstall more application.
The first one is somewhat risky. I really have a bad feeling about resizing partitions.
The second one is quite absurd. You are looking for more space so you can add more stuff, and keep the old stuff.
But either way, try to free as much as possible. My Ubuntu says I have 250MB free space after removing 600MB -- yeah, I myself is a WUBI user.