Oh lovely. I like the dramatic effect of using contrast in your layout.
If you shape the black streak more into an S, that will soften it.
The strong contrast you are using (light yellow blocks next to black, upper right corner and left middle especially) emphasises the black shape, which is cool. Now this might contribute to you seeing it as "bulky" - perhaps play with letting the black "flow" out into the other blocks a bit more, just to see which you like. (I like your approach right now, this is just a suggestion in case you are seeking to change it!)
All that said, one thing I've been finding with the layout of these is that the more I get my head out of the way and just play (with relaxing music on when I can), I end up with a layout that pleases me.
And it looks great already! Looking forward to seeing more!! :-D :-D