The compressed hold is a ship fitting that holds far more than its size suggests: containers bound in composite on a heavy frame, built in an Assembler. It adds more cargo room than any other fitting, and lowers the ship's speed more than a cargo bay does.
Its blueprint is locked behind research. An ARCHON unlocks it with a trialled cargo bay design of their own that has reached a set cargo gain. It is a large fitting, so only a ship with a large slot can take one.