I don't follow.
Way I see it, when it comes to Fantasy and (even moreso) Sci-fi you got two possible types of things (when we're talking about made up things like speciaes, technologies and so forth) - things that are "real" within the said universe, but realisticly wouldn't work (energy life forms, organic space ships, teleportation, etc...) and things that fit the universe and might even be possible (singular shields, laser rifles, etc..)
If you refer to 1 that it's OK, Zerg make "sense" within Starcraft universe....(just as Barney or Telletubbies make sense within their own.)
There are intervals in StarCraft where you see an image of a ship, a building, or whatever and it is depicted in a realistic sense as opposed to a cartoonish sense.
For an example of this, think of the cutscene where several marines drop off a nuke aboard a science vessel shortly before getting carved up by Zerg and subsequently exploded to death. The station they're on is massive, much larger than they are, but the science vessel ingame is merely a fraction of the size and far less detailed. Indeed, the version seen ingame is merely a
representation of an otherwise
realistically imagined object. See where i'm coming from?
It's not realistic in terms of being real, but it is drawn and animated realistically as it would exist in our world if it did indeed exist.
That is what i'm talking about when I say the game merely portrays realistic units in a cartoonish fashion, hence complaining about the units ingame not looking realistic is utterly asinine.