I mean, its the same with your primary stat. Each point increases your damage by 1%.
So let us take this very basic example so we can use round numbers. You have a weapon that deals 100 fire damage per swing. You put on an amulet with 100 str and no other stats. You now do 200 fire damage per swing (100 base, 100% multiplier)
With me so far?
You then put on a ring that gives you 100 more strength. That increases your damage by another 100%, right? But lets look at the actual numbers.You now deal 300 damage (100 base, 200% modifier). However 300 damage is 50% more than 200 damage. So that aditional 100 strength increased your actual damage by 50%, despite it on paper giving you 100% more damage (100 str)
Same goes for most of the other stats (except for crit that works differently, as you can't have a crit rate above 100%, possibly others).
So you now put on a ring that gives 100% Fire damage (OP). You now deal 600 damage (100 base, 200% modifier from str = 300 dmg, 100% modifer from elemental = 600). Follow?
Then you get mage fists. Increases fire damage by 20%, well you do fire damage, thats gonna increase your damage by 20%, right? No.
100 base, 200str modifier = 300 dmg, 120% fire dmg modifier = 660 damage. An increase of 60 from before, or 10%.
I know this might have rambled a bit but hopefully it illustrates that nearly EVERY damage stat diminishes as you acquire more of it. OF course, the imbalance comes in that items can have more of some stats than others; therefore some stats are still more valuable even after you've stacked up tonnes of it For high damage, try to dip into a little bit of every source (elite, elemental, IAS, %dmg) without stacking one too high at the expense of others. Even though IAS is kinda shit nearly all the time, if you have 1 bit of it on 1 piece of gear it will be a flat 7% increase in damage; where even a high roll of something else may not be as effective.