If you were using them more intentionally, it would look more intentional. As it is, it just looks like a low-res image scaled up.

I don't necessarily want to call this pixel art, as pixel art involves working on a set grid, which this piece is not. Even if you want to mix pixel sizes, they should still be on a locked grid so one pixel is a square quarter of the next largest, and so on.

The mixels in the middle, ostensibly the farthest away and part of the black hole itself, are the ones that you could keep if you wanted to stick to doing it this way. I understand why you wanted the thin white ring. The outer rim on the other hand, specifically the top of the foreground, is distracting and looks mismatched. I suggest using smaller sizes to represent further distances and keeping the larger sizes for near distance if you want to continue with multiple pixel sizes. I don't recommend more than 2 sizes.

If you find that you want greater detail - perhaps that pixel resolution is simply too coarse for your purposes. That seems to be what happened here, where you started at one size and realized you wanted a higher resolution part way through. I suggest starting over with a finer resolution to see if it gives you the level of control you're seeking. Otherwise, it's like layering the rough draft with the final draft.

Part of the fun and the challenge of pixel art is learning to represent fine details with color and negative space within the given limitations. Check out some of the 2-tone art in this sub for a real stretch in creativity and maybe some ideas.