There is nothing wrong with using such a large drive to put your OS on. Its just a personal preference to use something much smaller
and typically suggested, and here is why.
Anything that will access the whole drive at once will take much longer, i.e. Anti-Virus scans, defragmentation, formatting, etc.
Boot up times will be much slower as well.
Putting your OS on a smaller HDD will make your software and OS load much faster.
I'm currently using a 480gb SSD HD. Just to give you an idea of how fast it is;
My PC completely boots up in 11 seconds. That's from completely off to fully booted up ready to go with all startup software loaded.
Defraging my SSD HD takes about a minute.
Ant-Virus scans take about a minute as well.
The SSD's are a bit more expensive like @1stLt Robb said.
I'm just sharing my opinion as a computer nerd.