Originally Posted by MouseFink
IMO....larger OD valve springs provide more valve spring seat and open pressure.
Well, this opinion is not only useless but in many cases wrong. The pressure the spring provides is the pressure it provides- the OD is irrelevant. A larger OD should be avoided if you can though because it does increase the mass of the spring.
Spring OD, ID, material, thickness, shape, height, coil count, etc. all go into making the spring's open and closed pressure. Increasing the OD is one of the last things they will do to increase the pressure though as it increases the mass which can lead to surge. People spend a LOT of money on spring material just to keep the OD down while supplying the required pressures.