Sure, if your realm has 20% more gold than your enemy, all else being equal it will be 20% more powerful; but if nothing happens 3 months from now it will still be only 20% more powerful, and if you've found an edge by that time you can still win. They can't use that 20% advantage to become 4 times as big just by letting time pass.
More or less.
If a realm produces X excess gold (after maintenance and all) while another produces 1.2X excess gold (for a similar army and all else being equal), and they are at peace, then over say 20 weeks, the richer realm will have accumulated 1X surplus gold, which can then be used to temporarily boost the military enough to give it a considerable edge in the first battle and therefore allowing it to completely cripple the other realm in a few strokes only, while it would have taken much longer had they not waited.
Say X is 2000 excess gold per week (meaning the richer realm produces 2400 excess gold per week) , then at the end of 20 weeks the richer realm will have 8000 more gold on hand for military efforts.
Mind you, they have to actively be saving up their coins, which most don't. And still income on its own isn't quite enough, as it takes proper strategy, a good army (noble-wise), and proper diplomatic relations to win wars. I was just saying, there is an advantage to starting out richer and staying in peace for a while if you are planning a war.
