See, I find that hard to believe. Unless you can show direct evidence that sieges scale with wealth in such a way that extreme wealth immediately begets extreme sieges in the line of 'at least 10 macelords, 50 wrestlerlords, 20 hammerlords, 30 swordlords but most frighting of all is "20 goblinbowmasters"', then I've had quite a lot of very strange fortresses with over 200 million in wealth that only encounter moderately sized sieges if ever sieged at all.
I mean, during my testing for my goblin mods, I did have to test for siege strength too you know. It's why I mentioned, from experience, that even with millions in wealth, sieges won't get overly powerful without time as well.
I'm not accusing you of being wrong though. This is research. If I'm wrong, so be it. If you're wrong, it doesn't really matter as well. Just that we get the facts right.
My observations usually indicate that wealth and time only act as triggers to sieges and ambushes. As in, if your wealth and time pass a certain point, then they will come. However, only time and repeated attacks determine size. Hence, it is not uncommon in cheating fortresses to have ambushes in the spring of the second year. But you won't get a 100 goblin siege anytime soon.
That is my observation.
p.s: Hehehe, I can't help this one. The reason you'd be confused is, 99% of the time, time increases with wealth.