I'm agreeing with some of what @tombarrettillo has pointed out.
The helmet does seem a bit large, but perhaps that was intentional? It's hard to know without what your other work is like.
-The top foot placement also seems awkward to me. I feel like there should be some indication of a foot rest.
-The shadow feels a bit oppressive. I would think about lightening it up and introducing some subtle color into it, and adding some gradation to the background might make it feel a bit more polished.
This was an ambitious drawing to take on, so well done for seeing it through!