Just look at a graph of global military spending interlaced with time-periods of relative global peace. You will find a remarkable—but ultimately unsurprising—correlation. You will find for example that after the Napoleonic wars in the early 19th century, global conflicts reduced dramatically along with less military spending.

Pre-industrialization, governments used to spend upwards to 90% GDP on their military, and tiny countries existed back then as they do today. Today there are quite a few smaller countries (and a handful of medium sized ones) that don’t even have military at all. I think your narrative of smaller countries being snapped up by bigger ones is not a historic pattern at all.