First of all, air units in sc2 stack while they are attacking. If you control 5 phoenix and right click on an overlord, they will stack on each other as a circle with the radius of its range. Same goes to viking, banshee, muta etc. These stacking make the area unit where they stacked on became the "area with a high DPS". This means that on a 4x4 squares units in game, the dps will be much higher with air units stacking compare to grounds. For example you can see this through out the relationship between 50 mutas and 100 stalkers engaging assuming 100 stalkers will win 50 mutas in a straight up fight. The mutas could be stacked together causing an area with high DPS, This 1 single area then will engage a small part of the 100 area of non stacking stalkers and took out that stalkers area with minimum loses. This is because the rest of the stalkers are simply out of range.
another good example would be any relationship between range and melee units: marines vs zerglings, marauder vs zealots, roach vs zealots. As the number goes up to stack in a max range ball area(mean inside a group of unit every single unit could reach the melee enemy.), the range units becomes much more efficient compare to the melee as their area could deal more damage overtime than melee units.
Now if you still have any idea what am i talking about, continue below, else call me crazy and leave.
From that perspective, i can start examine some of relationship in Protoss match up:
+ Blink stalkers vs roach lings: as a certain point, the number stalkers will be high enough to fill up the choke that its standing on. This given the perfect micro, stalkers vs roach lings battle becomes the battle between phoenix and muta as you have seen in a youtube video in beta where Mutas constantly chasing a phoenix and get out ranged while the phoenix is moving shoot. As a result, 1 phoenix killed 200/200 mutas. This means that given 2 areas of similar DPS generated, the area with higher range and mobility will win the battle even if it has less DPS than the other by a small factors. This is how i explained +3 blink stalkers won vs hydra ling in a certain famous game in the last MLG.
+ marines, thor, and radius: the radius of units means a ton of thing: it cost less area to fit it in a ball. Meaning a group of marines always has a higher dps than a thor given that they cost about the same area to stand on. So if a group of undead marines got attacked by zerglings vs an undead thor vs same zerglings count, the marines will take out the lings first. This often explained how efficient MMM is in TvP match up: given an area to stand on, Terran can always fit more small range units than Protoss: an area to fit 1 stalker could fit 3~5 marines which cause the marines area has higher DPS. With the concussive shell upgrades to lower the P mobility (aka. no more micro), MMM demolished same cost gateway units based on their higher dps area and mobility (stim).
+ When you think about the definition of "area generating higher damage", it could be the opposite of AoE damage that we all familiar with: one is how to have more units in 1 area to be more efficient, one is to kill more units in a small area. This explain how late game AoE is very efficient vs MMM, fungal or ultra or storm or colossus, even a FFs early game.
I dont have much time to work on this but i think i post it anyway. At high level stuff like this really matter to theory craft out new build and i think i should keep it in my blog in case 1 day i forgot