I actually think your sense of proportion is pretty good. Some bits are off in the heavily foreshortened areas, and those require a lot of attention and repetitive practice to get right. But the muscle simplification on the arms and legs looks clean. Some of the poses could look flat because there are subtle angles in parts of the body that you have to make sure to capture. For example, the hips and ribcage could be leaning forward, leaning to the side, leaning back. I'd recommend you try to act out the poses yourself and be thinking about what position the parts of your body are in, how much pressure they feel, how balanced you are etc. Then go back and try to draw the pose while capturing what you noticed.
Hope this helps, sorry for the ramble
