They do hold almost six gallons. If you couldn't put in at least five gallons then you didn't get it completely empty, or you didn't get it completely full.
It takes a while to get the last gallon or so into the system. It has to drain into the engine past all the restrictions. There is a "jigggle pin" in the engine to allow coolant to flow into the engine when filling the system, but it doesn't flow much coolant.
Loren