It's true that 2:3, would indicate that the ratio of your strength to your opponent's strength (0.67), but for the chance of victory you would want something like the ratio of your strength to the total strength of both armies.

If forces are equal (1:1) you have 1/(1+1) = .5 (50%) chance of victory.
For the 2:3 case you have 2/(2+3) = 0.40 (40%)
For the 2:4 case 2/(2+4) = 0.33 (33%).
9:1 = 90%

You get the picture.

Having said that - I doubt it means much. I could make a case that it would be better to use the squares of the numbers above - I'll bet you'd win more than 90% of the battles you fought if your strength was 9 times that of your opponent's. So maybe 2:3 -> 4/(4+9) = 31%. In addition, it's never appeared to me that this ratio took any account of terrain effects which make a big difference in the outcome of a battle.

So, it's probably best not to try to translate this strength ratio directly to the chance for victory - its not clear how to do it exactly and there are qualitative factors to take into account.