Hi Boo,
Adding the additional phrases into the text and title tags of the page that is currently ranking #1 could help. So long as you do it in a natural way then there shouldn't be a problem. My rule of thumb is that if it sounds stupid when you read it aloud, then you've over done it.
If you add something into the URL you should be aware that you're effectively creating a new page so you need to weigh up the benefits of doing this for a page which already ranks well for a relevant term -horse buggies in this case.
It's fine to build links with the anchor text of the new terms (so long as it's white hat link building of course).
If the market for large and giant horse buggies is large enough to warrant a page that lists exclusively those products then that's personally the avenue I'd go down - create a new page which is tailored to help people looking especially for large and giant horse buggies.
Let me know if that didn't make any sense at all - I got disrupted part way through writing.