No, stud muffins, not what you are thinking.
Large Diameter Hose is exactly that - hose with a large Inner Diameter. The form I am discussing is specifically related to fire hose, and generally is used when referring to the largest fire hose - 4", 5", and now even 6".
There are two type of these: soft and hard suction (it just gets worse, doesn't it.) Hard suction is used to draft water from a pond, lake, swimming pool or other source. It works by removing enough pressure from the inside of the hose that the outside pressure forces the water up the hose and into the fire engine. Soft suction is used when transferring water from a pressurized source, such as a hydrant or another engine, and need to have the lowest friction loss possible, but still have some flexibility. This can not be used for suction, as it is not rigid like hard suction, and would collapse when the inner pressure was removed.
The biggest drawback to using these hoses is their immense weight. 4" LDH comes in varying lengths, but is most commonly in 50' sections. When under pressure, each section can weigh close to 1,000 lbs. 5" and 6" hoses usually are only in 5-10 ft sections.
And for those curious - back in 1999 we had a fire, and one of the fire trucks trying to get to the scene violated the golden rule. When they did, the supply line separated at a coupling right next to three firefighters. When it broke, it picked all three of them up in full gear and threw one of them nearly 20 feet.
Not a good way to keep them as your friend.