platinum casting issues!

Howdy, I sought help a couple of weeks ago for an engagement ring I was making, by carving in wax and getting it cast in platinum. Well, it's just arrived back from the casters, and I've discovered it's too small. I thought it would get a little larger through the casting process, but it seems not to have changed at all, it might even be smaller!

I'm trying not to panic, since I know there will be a solution to this, I just don't know what it is yet. As I said, the ring is platinum, and this is the first time I've used it, so a bit more complicated than silver. I also don't have equipment for working in platinum, which is partly why I decided to go down the casting route. I'm happy to buy a few bits for polishing and filing, to keep exclusively for platinum though.

So. I'm hoping someone can reassure me that I haven't made a complete mess of this (the carving went very well, and I'm pleased with the design) and advise the proper way to finish a platinum casting (and make it all shiny and lovely rather than the dull finish it has now) and how to make it a tad larger without inserting more metal. It's thick enough to be able to take some material off the inside of the ring, if this is possible.

please help!