How? Lollipop, the first Google hit for "Android OS size on phone" (https://android.stackexchange.com/questions/118896/how-much-...), seems to be about 5 GB. (Maybe you don't run Android, or maybe a very old version?)
I've seen a phone like this running Lollipop. The best part is when it decides to auto-update all the built-in (Google) apps, collectively consuming a total of 1-1.5GB of /data, at which point Google Play refuses to let you install any apps because you do not have enough free space available.
Oh, so you read bigbugbag's comment (https://news.ycombinator.com/item?id=15049992)
> > wow those people are lucky and high on consumerism, my phone had 2GB.
as saying that it's 2 GB after the system installation? That seems to make it a strange response to itg's comment (https://news.ycombinator.com/item?id=15049947)
> > > Considering how may people still have 8GB/16GB phones, I doubt it. On iOS sometimes you have to delete apps, especially ones that build up large caches like Facebook.
which is clearly referring to the pre-OS available space.