I don't know about others, but I think there is a difference between, thicc and curvy.
When I use the term curvy, I usually think of the curves of the body being exaggerated, or being the focus.
Whereas with the term thicc, I usually think of the focus is that you can notice the body has thicker proportions (it can just be the legs, not the whole body), wich this does often lead to more curves as a result.
Thick can make curves, but curves do not mean thick. kina like a square is a rectangle, but a rectangle is not a square.
With my practice with trying to make thick characters, at least the characters I like to make thicc, the legs and butt work just fine, but the rest of the body doesn't match. So instead of thicc, instead I think curvy. Or as you may say, body built.
having a built or bolted body is fine, a different style choice and not something i have experience with as I try to avoid such style.
As for the breasts, again it follows the theme of the rest of the upper body, a vibe of built or bolted, or one could argue artificial. To me, because the key shape or simplification I see is just that they are big around round, that they are artificial or bolted on.
If you wanted to express that the boobs were big, but not necessarily bolted on, then to avoid the artificial feeling they may give off, you would need to have another shape or line to be obvious, and that would be the top curve of the boob. Basically, to avoid the artificial feeling, the breast will need weight, which it looks like you've already know how to do from your other images. So I assume you were trying to make a more built body this time.
But assuming you were just trying to make the boobs look big, then the main shape there needs to be is, instead of spheres, a sphere with a slope on the top, like making the breast look like drooping water balloons. Boobs being round is a common way to show that they have volume. It's fine to keep the bottom/front of the boobs round. But the tops being round indicates being pushed up, or if there is nothing pushing up the boobs (clothes or an arm underneath) then the breast feel artificial, being over filled.
Now one challenge I find with making bigger boobs is that, to try to keep breast looking natural, they need this weight. But the bigger one wants to make boobs, the more the boobs have to be lowered down so that there is room for this sloping area. This can results in the boobs being low, which you don't want, you want them on the chest, and having them low may obscure the body, which is a detail of the art you may not want to loose. I took a bit of inspiration with how big anime titties are done, but I am not going to tell you what is the ultimate solution because I think trying to find the compromise between sag and no sag, trying to figure out how to make breasts look big without looking incorrect, and do so in a way that the artist finds comfortable, attractive, and repeatable is one way an artist can find their style. Thus there is no best boob design... or at least I don't think so.
