Cunobelin is your man for this. I recall him saying that the EN standard used over here in Europe is not nearly as stringent as the Snell std required in the US. Spesh are supposed to be Snell certified, apparently.
...enters stage left!
Snell standards and EN1078 are worlds apart.
RIght form the start where the prototypes or specific batches can be tested under EN1078, whereas Snell always takes helmets off the shop shelves - you have the confidence of knowing what you are wearing is what is being tested.
There is an
American helmet site which discusses this in more detail than you may need.
Secondly - weight.
This is not clear cut.
There is an argument that lighter helmets have less absorbent material than heavier ones, additionally there are claims that the remaining material is denser to maintain form and structure, Again decreasingt he ability to absorb the energy of an impact. Some companies (Specialized) are playing with tow density foams to enable structural integrity but enhance the energy absorption.
There will always be a trade off between the weight and the ability to absorb the impact energy
Having said all of that