I agree that the mountains should be reduced in size. That should help with making the world look larger. Also, some variation in the form of the mountains would help create some visual interest.

Another thing that might help would be to reduce the size and intensity of the brownish glow around the land. The present width might be contributing to the smallness effect and it is a bit distracting for the present.

As to the rivers, perhaps drawing them not as gaps in the landmass but as simple lines would be good. At the general size of a world map rivers aren't very big.