Basically there are two signal paths; once going through Xop1, R1, R2, R4, R5, and Xop3 and the other going through Xop2, R3, R2, R6, R7, and Xop3. Each of the signal paths has its own gain. Anything that causes these two gains to be different will hurt CMRR. R2 is shared between both of them, so it doesn't affect CMRR. But any of the other values will matter. If you want to derive the exact relationship, just calculate the gain from the common mode source to the output as a function of the resistances. Shouldn't be too complicated.