It would indeed be hard to make sense of it if Socrates were claiming that pains seem lesser and less intense when compared with pleasures, when he is claiming that pleasures seem greater and more intense when compared with pain. It would also be inconsistent with what Plato says about the mutual intensification of juxtaposed pleasures and pains in Republic IX (at least as I understand it): 586c1-2, for instance, suggests that both pleasure and pain appear more intense when juxtaposed with one another. Moreover, 584a7-10 and 584e8-585a5 suggest that the effect of juxtaposition with a contrasting experience is symmetrical in the case of pleasure and pain.
My suggestion is to understand 'the opposite' differently: we are to understand that the intensity of pleasure and pain can be placed on a continuum, such that maximally intense pleasure and maximally intense pain are at the opposite ends of this continuum, with the neutral state in the middle. On this understanding, pain appearing more painful and intense could be understood as the opposite of pleasure appearing more pleasant and intense. This use of 'the opposite' would be like its use in saying that on certain economic policies, the rich get richer and the opposite happens to the poor, meaning that the poor get poorer - each becoming more of its kind.This would get Socrates to say something sensible, I suppose. Nearer pains might appear more intense and nearer pains might do the opposite, by appearing more intense too.