You show a decent understanding of abstracting simple lines and forms, and draw clean and readable poses. I suggest you try to include anatomical observations next.
I think the next step forward for you is to understand how the ribcage actually looks and functions in the chest, and try to "see" it in a reference before drawing. Short description: a flattened egg-like form, with the "pointy" top ending between the shoulder joints, just underneath the neck, and the bottom cut off along the line of the lower ribbs. That cut-off extends on the back down to about a hand-width above the hip, about two handwidth on the front, where the belly is. Understanding the ribcage is also the first step towards observing shoulder and breast muscles, as they are mostly what obscures the ribcage. Be aware, overestimating the distance between hips and ribcage is a very common mistake, which tends to lead to unproportionally long torsos.
In your drawings the chest area has a tendency to be a bit slimmer than natural and looks like a simple tube. Finding the ribcage would add more substance to the construction.
Btw, female breasts are best imagined as formed like an inverted heart shape, hanging over the ribcage with the tip attached underneath the throat. A good shortcut to look for is the line underneath the breaths. Finding that line and the center line of the heartshape is often more indicative than starting with sketching out one of the spheres and then finding the other one. In constructing the foundation consider even ignoring them, as they don't actually influence the pose a lot, but merely feature in drawing the outline.
Also, check out proko.com for better explanations and examples.