The coefficients on the cobb-douglas utility function equal the proportion of total income that will be spent on that good at the utility maximising point.
im pretty sure, but i haven't checked, that something analagous will be true for the production function if the coefficients add up to 1.
PS: you asked about returns to scale ofa utility function. i dont think the concept of "returns to scale" really has any meaning when you are talking about utility functions. The distance between two utility values has no specicial significance, all that matters is which one is bigger (not by how much).