I would love to do an experiment with 2 twins. Twin A does nothing but 2 high set arm workouts each week consisting of both compound and isolation exercises for both the biceps and the triceps an no work for any other bodyparts. Twin B also does 2 workouts each week. One of them consisting of heavy squats followed by 1 set of heavy 20 rep squats and the next workout consisting of deadlifts done in the same fashion. If each twin were to receive an ample diet I doubt there would be much, if any, difference in the increase in arm size or strength for that matter.
The above responses are very correct. You MUST work everything to get big, otherwise, you just stagnate. When I sais bodybuilders such as myself do isolation exercises, I mean we use them, but not exclusively. They are another tool, and a good one. I do isolation exercises for ALL of my different muscle groups so I stay balanced. In addition, I do compound movements like squats. The key is to make sure no one body part gets left out - if you use isolation exercises (and if your goal is to get big rather than just get strong, you SHOULD) make sure you use a workout split where no muscles get left out. Its not necessarilly true that you have to do exactly the same number of sets for opposing muscles because some muscles respon best to volume work while others do better with less sets. This varies from person to person. The key is, make sure EVERY muscle in your body gets attention.