In general, there are two forces - selection for bigger males if there is competition between them for females, and fecundity advantage hypothesis - larger females produce more offspring. Personally, I don't see any reasons for males in ravens to be bigger - they are monogamous, do not compete for females or resources...
Corvus corax are relatively large birds that display monogamous behaviors. Their eggs are generally preyed on by larger birds, such as owls and eagles. I personally think that the males tend to be larger in order to support its female counterpart in protecting their young. Their larger size also prevent predators from preying on them, thus increasing the chances of being able to protect their offspring.
See also the information on the species contained in the next scholar book:
CRAMP, S. & C. M. PERRINS (eds.) (1994). The Birds of the Western Palearctic. Vol. VIII. Crows to Finches. Oxford University Press. 899 pp. + egg plates.
(Pages 206-223 are devoted to the Northern Raven Corvus corax)