Despite the title’s provocative question and definitive answer, it’s not that clear cut. The truth is, in any design, there is going to be a budget. That balance is usually between gross floor area and detailed design. Some rooms are really about gross floor area or volume, others need to be compact but highly detailed.
There are extremes on both ends of this spectrum. A tiny house is a highly detailed, engineered design that minimizes square footage by utilizing every available cubic inch of the available space They often make the same area serve multiple functions. The downside to that is you really don’t have much room or privacy. A lot of these homes are partnered with large outdoor spaces that provide ancillary public space for entertaining or exercise. The other end is something like a McMansion which trades small details and engineering to provide something between 3,000-5,000 square foot of living space in a mass produced package. This does mean that you get a lot of house for your money, but it’s usually not efficiently layed out for it’s intended use.
Custom Middle Ground
In a custom designed house, it’s a balancing act. Some rooms really focus on great rooms, foyers or large entertainment rooms. These rooms, intended to hold large numbers of people or being impressive, are often about providing floor area or volume (or both.) But gross size for walk in closest, kitchens, and bathrooms is less important than how they are laid out. Having a giant great room full of trim and paneling and small nooks and highly detailed design gets expensive quickly. Having that level of design in a master bath or kitchen though is more manageable and more functional in the long run.
When designing your house, remember to take this balance into account. If your budget is tight, focus on the parts most important to you first. This could be achieving a style or look. It could be having enough room for a growing family. It could be having the nicest master suite you’ve ever seen or the best lit space for entertaining possible. The point is, make sure you know what’s important to you. Find out what can be improved easily later, and don’t blow the budget by building a 400 square foot closet that doesn’t lay out efficiently.