Maybe I'm missing what you're saying, but I think that by itself, the bare statement that "sometimes empires get larger and sometimes they get smaller" is about

Maybe I'm missing what you're saying, but I think that by itself, the bare statement that "sometimes empires get larger and sometimes they get smaller" is about as useless as saying that stock markets fluctuate? But the reasons why it happened in various cases are often worth reading about. That's why we read history.