Deadlifts and squats are primarily lower body exercises and will not increase the size of your arms. Depending on the type of deadlift or squat that you do, your arms will sometimes work to hold on to weight or to stabilize your body. But this amount of work is not sufficient enough to build muscle. Increasing your arm size will require a better upper body approach during your workout.
Squats work your entire lower body, from quadriceps and hamstrings to glutes and calves. A closer stance in the feet will target your outer hips while a wider stance will get the inner thighs working more. The only way your arms will work slightly while squatting is if you are holding on to weight by your sides, with a rack on your shoulders or a bar in front, doing a front squat.
Deadlifts also work lower body, targeting mainly the hamstrings, glutes, calves and lower back. Your traps, lats and forearms also work, but not to the extent that your lower body is working. The arms only come into play by simply holding the bar. Your lower body is typically stronger than your upper body, since it houses your largest muscles. Therefore, your legs may be able to deadlift more than your arms can handle at times. Some people use straps to keep their grip on the bar. Without straps, you will target your forearms and make them work harder.
Building Bigger Arms
Bigger arms will come from being bigger overall. In simplest terms, if you are underweight, you will never have big arms. Your muscle mass throughout your body needs to develop as a whole, meaning compound exercises that work multiple muscle groups at once will be better for bigger arms than isolation exercises, like bicep curls and triceps extensions.
In addition to your squats and deadlifts, which will build overall muscle mass, begin to do curls, bench presses, seated rows, shoulder presses, pushups and pullups. Your arms need to fatigue in order to begin building strength. Just relying on squats and deadlifts will not make your arms reach your desired size. Start by working your arms two times a week, being careful that you don't overtrain them. Your muscles need rest in order to grow.