I'm trying to install UBUNTU using Windows Installer, though its written less than 500MB in the instructions page, the actual installation size is mentioned to be 18GB with 5GB being the minimum, please help me by explaining the difference between two if any and also since I'm on a limited data plan, any download of more than 1Gb will make a big dent on my pocket. Please help. Check instructions to install here.
The 500MB download size is correct.The 5GB installation size indicates of the hard disk space Ubuntu is going to occupy. So no worries about your data plan.
To further explain: it downloads a .tar.xz file from http://releases.ubuntu.com/12.04.1/ubuntu-12.04.1-wubi-amd64.tar.xz which is around 486 MB. However expect same file for 12.10 around 536 MB.