Pillager,
Technically two 3 strength units = one 5 strength unit. 3 strength unit chance to hit is 3/20*15/20=11.25%. 5 strength unit chance to hit is 5/20*17/20=21.25. So the 5 strength unit hits twice as often and hence you need two 3 strength units to equal one 5 strength unit.
Personally I see a Giant kill 2 elves/Hv infantry/Lt cavalry all the time assuming no terrain/city bonus's in play.
Now, I also have seen the defeats you've noted. In Beta3 there is a smoothing algorithm added to prevent outlier outcomes (things less than 10% likely). But 8 Dwarves beating 6 Giants is well within the realm of possible even losing 4 dwarves is probably within the 90th percentile.
Now I will say this. The random number generator here seems to behave different than the real game. That may have to do with how often its seeded, the fact the server is running 24/7 vs a temporary game etc. Over long numbers of rolls it converges correctly but in short term numbers it appears to be pretty wild with the results (In another thread I manually charted 1000 rolls in the replay screen in one of my games).
Also there are some days when I can take a turn in 6-7 games and get no hero offers. Other days I get an offer in 6 of the 7 games. I tend to play my turns on days when the game is handing out heroes
KGB
P.S. 3 strength units should mop up 2 strength ones. They are more than 50% better as a unit. So an 8v8 battle should on average leave 3 units left.