In CK3, there's a system where within a battle, there are a bunch of advantage rolls that add a bit of extra strength to one side or the other. The trouble is that because each battle consists of many such independent rolls, the variance washes out and the rolls barely matter. Where do you strike the balance of variance with fronts and battles? Does any individual battle really matter if there are going to be many of them before the front is won or lost?
A 100 vs 20 front is a "sure thing", but what ranges of strength ratios do you envision not being a "sure thing"? In a vacuum, how should we think about the odds of victory in a 60 vs 40 front, for example, assuming all else is equal?
All other things being equal and taken as an average, intuitively I'd say the odds of a 60 / 40 Front should work out to about 70 / 30 with this system.
With a traditional system where all troops in a stack are involved in every battle, there can still be a number of variables that smooth out the squared effect of numeric advantage (like combat width, for example) but what it comes down to is usually the dice in that
first battle. If the first battle favors the underdog, the post-battle numbers might be 40 / 35 and at this point all bets are off. But if the first battle favors the advantaged side, it might be 55 / 20 and then the rest is just a given.
The other aspect of relevance is that our peace system doesn't necessarily require a front to be "won or lost" - it can sometimes be enough to have made only a partial incursion to force a peace deal, without wiping out the other country's whole army. So even if your 60 / 40 has been reduced to only 30 / 20 and not the big win you hoped for, if you've made gains in the process that might be enough to get what you want. More on peace in a few weeks.