区别就是两者都是动词cut短语，但意思表示削减时是一样，其他具体的不同如下
cut back中文意思是削减修剪（美）倒叙急忙返回
Look for expenses that can be cut back， and don't give up in the face of high fixed expenses.
cut down中文意思是v. 削减砍倒杀死删节胜过
A vandal with a chainsaw cut down a tree.
