The cells themselves are roughly the same size whether you are in reception or on the mainline. Do not expect a meaningful difference in square footage between the two. What changes is everything around the cell rather than the cell itself.
Reception feels more restrictive because it is. The movement is controlled, the programming is limited, the privileges are reduced, and the population is constantly rotating as new arrivals come in and get processed out to permanent facilities. The environment has a temporary, unsettled quality to it that the mainline does not. Once you hit the mainline and get assigned to a permanent housing unit, there is more structure, more routine, more access to programming and recreation, and the social environment stabilizes in a way that makes daily life more manageable even though the physical space is similar.
On food, prison food is better than county jail food in most cases, though that is not saying a great deal. County jail food is often the lowest rung of correctional food service, under-resourced and poorly prepared. Prison food at the state and federal level tends to be more consistently prepared and nutritionally adequate, closer to what you remember from a school cafeteria than anything you would choose voluntarily. It is not tasty, it is not exciting, and the portions are designed for sustenance rather than satisfaction. But it is regular, it is consistent, and it keeps you going.
Most inmates supplement the facility food with commissary purchases, which is where the real quality of life difference comes from. Having money on the books to buy food through commissary makes the daily eating experience significantly more bearable than relying on facility meals alone.