Mysql:将int字段设置为无符号可以节省大量空间。为什么?

I was under the impression that setting an int field to signed vs unsigned doesn't save any space. Today I set some columns to unsigned, as they don't ever store a negative value, and noticed a 15% reduction in table size. Some of my tables weren't reduced in size when I set their int columns to unsigned, some tables were substantially reduced in size. Anyone have an explanation for that?I was under the impression that setting an int