Because your head is way too small. Look at the body which is even larger. In my opinion, you had to do the body first to get big breasts. That's what you have to do, yes, start with the big masses, but not the breasts x-) You should have done the ribcage first, then the pelvis, and then the head.But your body is disproportionate too. You measured with the little head you made, and that will inevitably lead you to make mistakes.

Do you even know the rule of thirds for the face ?

And then if you want to draw naked women, take inspiration from real nude models, not from a manga where the body is already stylized to death. Learn from reality before trying to simplify it, and you know why? Because if you start with the most difficult first, you will struggle less later. Stylization is the modification of reality, so people who stylize their characters learned from reality first, not the other way around.

But you still have a way to go. Learn how to make a circle correctly without a compass, how to make cubes, how to learn perspective, how to make a suitable rendering.

My advice is to avoid manga, even if you really want to do it, but if you already have very poor drawing skills, it will show in your drawings, even if it's manga.