Years ago Gnat was unable to be buffed because it was on the cusp of being an OP rush. Gnats were fairly accurate and a single one had the damage required to stun one non-heavy target indefinitely. If your opponent only has five units that are capable of killing Gnats or building units that can kill Gnats, then a Gnat rush only had to have five surviving Gnats to essentially end the game.
This was solved by giving Gnats a ridiculously high spray angle. Now it takes two Gnats to fairly reliably lock down a tiny unit such as a Glaive or Lotus, and three to keep it disabled almost permanently. Gnat rush is unlikely to end games against anyone who has built anything beyond their first factory because it takes so many Gnats to keep everything stunned.
The spray angle isn't really meant to affect large units, but it has nice side effects. The uptime of a Crab against Gnats is not just depend on whether one Gnat exists, it depends on whether there is one, two, or many Gnats focusing on the Crab.