ㄱ) 감소하다 vs 증가하다? = increase/decrease (somewhat formal terms used a lot in statistics and science). ㄴ) 줄어들다 vs 늘어나다? = similar to 감소하다/증가하다, but also means physical expanding/shrinking.. For example, 줄어들다 can mean an item of clothing shrinking, "옷이 줄어들었다"), and 늘어나다 can be used for expanding in size or length or weight ("체중이 늘었다/늘어났다"), etc. 감소하다/증가하다 can't describe such things as easily (you can say 부피가 증가하다/감소하다, though, although it sounds very formal).