The issue really falls around a math and physics problem, assuming all things being equal (same bullet design). Additionally their are two things to evaluate. the properties of the bullets and the properties of the material to be penetrated. On the bullet side, the mechanical energy(potential + kinetic) and its penetration property(will it hold together) is key. On the material side, the mass, tensile strength, and the elasticity or lack there of is key.
Small and fast is great until it hits something that is hard or harder with mass, then it dumps most of it energy very fast. Concrete is hard and it has very little, if at all, elasticity. However, those cinder bricks that were used, have air pockets and, in the video the OP provided, the brick was also hollow in the center. Fill the brick with sand and you'd likely see that the 5.56 would not perform very well at all. However, due to increased in kinetic energy, the 7.62, would fair much better against the same type brick.
In the videos with the steal armor plating - those are far from scientific evaluations and would not hold up to even slightest scrutiny as a valid evaluation, in my opinion.