system
Given the following post, respond as if you're commenting on Reddit:
human
[Starcraft] How big are Protoss Interceptors?

So carriers are basically giant floating factories but in a way they are wasted. Why can't they have stargates inside the carrier? Why not have a probe warp in units or buildings inside the carrier, why not build scouts or cosairs instead of interceptors?

Anyway, assuming warp tech does not work on carriers, or for some reason they only have shields and no counter offensive measures, why can they only hold 18 interceptors? A carrier is freaking huge.

This leads me to believe that interceptors are actually the same size or larger than scouts, they are just misrepresented on the game screen for balance.
gpt
(The limited number of Interceptors in game is just for gameplay balance.) The "real things" would most likely hold thousands, if not tens of thousands of interceptors depending on how big the carriers are. We know that Protoss send them out in "swarms" after their enemies, which implies FAR more than the paltry handful you can command.

Also, we know at least some of the space inside the carriers is dedicated to their giant purification weapon that can turn an entire planet into glass. The rest is most likely taken up by manufacturing facilities for more interceptors and repairing the ones that come back damaged.

The interceptors themselves are probably just a little smaller than scouts, since they still need to hold engines, weapons, computers and sensors. They just need to sacrifice a pilot's seat and controls. 

The big advantage of using robotic interceptors over piloted craft is that they can be stored much more efficiently. Piloted craft need space not only for their pilots to move around the hangars and get into position, but also living quarters, medical facilities, etc. No pilots means no space sacrificed for creature comforts. Which means more storage for interceptors and/or maintenance.