How to make character-mesh deform naturally when animating? I have created a character-model in Blender. After rigging and skinning it, I noticed that it deforms in a undesired way. Everything moves pretty much as expected, but the mesh around joints deforms weirdly(Screenshots below).

Why is this happening? Is it due to a low amount of vertices? What can I do to create smoother deformations that look natural?

Character in T-pose:


When his arm is rotated downwards. Look at how thin the upper part of his upper arm is:


Up close:


His upper arm is very thin at the shoulder area, but has normal width at his elbows.
Add edge loops on your mesh  at both sides of a bones joint.

Ctrl+R with the mouse cusror on an edge, click to add one (roll middle mouse button for more than 1), slide mouse to place it, click when it's where you want.



When done, parent your armature again with automatic weight (you'll play with weights later :))