Fewer and fewer has to do with things you can count or has a plural form(like money, houses, cars). For example- Fewer and fewer houses are being built these days.(you could not uses less and less or smaller and smaller here) Less and less has do do with things you can't count or does not have a plural form(like money, time, music). For example- I listen to less and less music these days because I'm too busy.( you could not use fewer and fewer or smaller and smaller here) Smaller and smaller has to do with numbers or distance from zero. For example- The length of sand at the beach is getting smaller and smaller each year.

less: 指程度或强度。（也可以指数量） smaller: 指大小。（也有时候可以指程度或强度） fewer: 指数量。（如果已开始只有一个东西不能用）