Chest deformation low poly This is my low poly

Here a shot 



Don't look hands, i'm finishing it these days.
I would like to understand one thing, making me crazy.
I'm trying to check if my low poly was made well for animation (topology of vertices, the shape of muscles in particular the back the shoulders and the chest/pectorals)

Now it seems working all pretty well except one thing: PECTORALS and AREAS NEAR THEM

Here the situation



If i move my upper arms down the deformation is this one



Now it isn't so impressive but the mesh of chest goes inside the shoulder in point 1 and in point 2 it deforms inside the body. I have tried weight paint but i don't think it's the definitive solution. I have doubts for my mesh and vertices.

Now my question is: 

1)Is a problem of topology? (i have followed a very useful tutorial for doing that in particular this picture) Too few vertices? I think 5800 for a low poly was too much^^

2)Is a problem of my mesh: did i make pectorals too bad, too large or something near the chest isn't right?

Can someone help me?? 

Here the link of my file
That should be a weightpainting problem.
As far as I can see you have more than enough mesh for the deformation in that armpit area and your geometry looks clean, too. 
Weightpainting can be a pain in the butt and takes a lot of time to get it right.
I never weightpainted in Blender, but usually 3d programs can display the weights in different colors so you can see more clearly what's going on with your weights.

Hope this helps you a little :)