Welcome!

That's a though challenge, I don't know what would be the best way to approach it.
There are two main things I notice that are different between the two trees:

Yours seems to be made out of 4 'balls', which all feel equally close to the camera, which makes the tree look quite flat. The Densetsu has some more variety. There are 4 main blobs, but also a bunch of smaller secondary ones. Also, the left blob clearly catches all the light, and the right blobs have a strong black edge, which gives the whole thing much more depth.
One way to start with the construction of this tree is by drawing actual shaded spheres and only do the texture later.

The second thing is the leaf structure. It seems almost magical, but every tiny cluster of pixels looks like a leaf on the original. I think it mostly has to do with the shading on the bottom and right of each little cluster of light. Your pixel patterns are similar, but as a whole they feel very random. Painting each single leaf takes a lot of effort and attention to detail, but that is definitely what they did on the original.

Keep on studying the original and good luck!