Hello and welcome :>
I'm not really good with harsh critiques but I'll give it my best.
You do tend to make your legs too small. The heads also seem bigger, but that could be stylistic, I'm not sure what you're going for.
The third picture is a good start, I'm not sure however you're using perspective or if the legs are accidentally to small. If you were using perspective, the gap between her thighs wouldn't be necessary. If you weren't using perspective, then her legs are a bit short. Superhero's legs are generally 2/3 of their body. Also her crotch is a bit to far to the left.
In general, as with any pencil drawing, try to make your pencil marks to add form. That is, when you're shading, don't just randomly sketch everywhere, make the lines follow the 'grain' of the form. I find it very hard to explain this so I'll just show you :>
This image is just one example of what I mean, but you can google up hundreds. Notice how the pencil strokes are not at all random, they are placed in a way that is logical and follows the form of the skin. I'm not sure if that was coherent enough, but if you have any questions ask.
Apart from that I think you're on your way. Keep practicing and get out an anatomy book every once in a while to brush up.
Good luck!