Hi, looks nice!
First thing that stands out to me is that there’s a lot of free space between the muscles.
If you take a cross-section view (change the camera clipping plane) I think you’ll see voids between the outer muscles and the bones as well. For best results you’ll want the muscles/bones to all feel packed together 🙂
I think a deer is a great choice for a character to do, looking forward to seeing how it turns out!