The only way that the physics could be the same in HL2 and in CS Source is if the guns did different damage
Damage is not the same as physcics push.
Both games use the same physcics engine, but in it there are different options to chose. The reason why that the physcics in CSS is different is because it is multiplayer and there you need to think of lag.
Most of the props in CSS are prop_physcics_multiplayer props because they require the least bandwidth, this is the reason why you just bounce of them. You(the mapper) can also chose to make them prop_physcics and thereby having more realistic physcics, but also more lag.
Btw. try shooting something moveable with the gun and the shotgun in CSS. Notice how the gun can move the things, but not the shotty. Someone fucked up the mass impact values:D
I think the shotty has the same amount of force as any other gun but its spread over a wider area. This reduces the amount of "push" in any one area so the shotty in CSS actually works in the opposite manner to real world shot guns. I think its just easier to code that way. As to why shotty doesn't move the small objects I think thats becuase it creates a spray and you have to be lucky to get part of that spray to hit the object.
Just my theory