I found Roverandom more fantastical than the Silm. The Silmarillion reads much more like a history than a fantasy story, so I concur with peregrintook that Roverandom is highly imaginative.
And it has a much more satisfactory ending, if less invovling of the reader.
It's more imaginative in the fact that it's random (Hence, I think, part of the name) More perculiar things happen, like Rover growing wings, or sand wizards. (That makes me wonder if it's a derivitive of a sandwich) That's because it was a book written for a child. A child cn accept these events and delight in them, whereupon they would never comprehend the Silmarillion.
I guess different people have different opinions on what's fantastic or extreme. I personally found Roverrandom fantastical in the sense a child would. Children's stories often have over-exageration and are usually completely made up of fiction and little fact. If you consider that extreme fantasy then I see why but I personally find stories which expand greatly more fantastical int he sense that they needed more imagination.