Those client lists are a bit unfair to compare: Atypon lists large publishers (Elsevier, IEEE, Oxford University Press, Taylor & Francis, ACS) while HighWire's

Those client lists are a bit unfair to compare: Atypon lists large publishers (Elsevier, IEEE, Oxford University Press, Taylor & Francis, ACS) while HighWire's list has a lot of individual journals (Journal of Early Childhood Research, Monthly Notices of the Royal Astronomical Society: Letters, etc.)