I remember seeing this on PixelJoint; looks pretty nice!
You're right that the giant could use more detail and contrast, but there are some other things that have room for improvement as well. I made a really messy edit to illustrate:
Here are some random thoughts to go with it:
-I added the extra contrast you were thinking of by both brightening a lot of parts and darkening others. There are still some sections I think could use more brightness because of how close to the lava they are, though.
-The lava in the giant's hands almost look like rope because of the way they're flowing, and the whole image is kind of static. Add some more motion!
-The giant's torso is basically one long tube. Try varying the width a bit to make the silhouette more interesting.
-You could stand to add something in the upper half of the canvas, it looks pretty empty as it is now.
-The palette has a lot of colors close to each other in value; you might want to reconsider which ones are actually necessary. Additionally, a secondary light-source could help the image pop more (I chose blue because it complemented the red/orange/yellow pretty well -- looking at it now, it's kind of similar to your avatar, haha
).
-The scale of the giant is a little fuzzy. I want to think it's huge, but my brain isn't entirely convinced. Adding something to compare the giant to would help.
I'm sure there's more stuff I'm forgetting, but you're probably already bored of reading this.
Hopefully this helps you a little!