star trek - Why are quarters on the Enterprise-D so spacious?

Watching TNG I noticed that nearly every crew member has very spacious quarters, which is something that does not seem practical on a space vessel.

Today's aircraft carriers house 3 officers to a room and 8 or more enlisted men to a room.

Why are quarters on the Enterprise-D so spacious and is there evidence that lesser crew members had more confined/shared living quarters?

Answer

Why the quarters are so spacious;

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  Ship space isn't at much of a premium. The ship can readily hold ten times the present crew when the need arises (for evacuation missions and emergencies) which means that for most of the time, the ship is 90% empty.

  
  


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  The Enterprise is not a military vessel. Their primary mission is one of peaceful exploration. In addition to there being a considerable number of civilians on board, the crew are not expected to maintain military discipline in their quarters. Providing them with spacious accommodation would obviously lead to less stress and a happier and more productive crew.

  
  

The TNG Technical Manual has this to say on the subject;

Starfleet believes that providing comfortable living quarters to all crew and attached personnel to be of primary importance. Indeed, living accommodations are one of the most visible displays of Starfleet's commitment to caring for its single most important "system," its people.

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The living areas of the starship have been designed for maximum comfort and safety while the crew is conducting a mission. Long-term studies of humanoid cultures have confirmed that as each race embarked upon permanent occupation of space, large personal living spaces had to be established, especially on early sublight expeditions. The Enterprise allows for some 110 square meters of living space per person, in addition to community space and the areas allocated to purely working functions.

While some engineers on the Galaxy Class Project questioned the relatively large size of the vessel, opting for a smaller, more efficient design, it was conceded that the large size provided a greater number of mission options, given the changing social, political, and economic conditions in the Milky Way.

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Do lesser crew members have less space?

Yes, crew quarters aboard the Enterprise-D (at least for Starfleet personnel) are assigned and sized according to rank. In the episode TNG : Lower Decks, we learn that enlisted crewpersons are required to share quarters until they reach the rank of Lieutenant.

LAVELLE : Promote me, please -- so I can make Lieutenant and have my own room.

TAURIK : If you aren't happy sharing quarters with me, then you should put in for a new room assignment. Just in case you're not promoted.

Above the junior ranks, Senior officers are provided with staterooms and at least one extra room for work/study. Picard and Riker use this extra space as separated visiting areas, Data uses his extra room as a private workspace and Troi uses hers as an informal workroom with her famously messy desk. The ship also contains "apartments" for officers who choose to bring their families on board, comprised of two normal sets of quarters combined into a single living area.