The performance difference is pretty subjective. It's not huge, but it's measurable.
When you're buying an SSD the price per byte is very expensive, so you'd get a relatively small drive, perhaps in the 40GB to 80GB range depending on how many programs you expect to install on it.
If you're buying a hard drive for the OS, then you don't really have a 20GB choice any more (they don't make them that small these days), and the price differences between, say 500GB and 1TB (and even 2TB) aren't all that huge. So you might want to buy a larger drive based on the idea that you'd be able to re-use it in the future (a larger drive will be useful for a longer period of time than a smaller drive).
Another factor is that if you buy a large...