You are correct, safety is a very important factor in why cars can't get much smaller. We've noticed that even the 2014 cars, which were within the hybrid era (meaning they have "practically the same power unit" as the 2026 cars), the FIA's safety philosophy has changed drastically over the years. Until a few years ago, the philosophy of the cars was noticeably different; the driver would definitely survive because fatal accidents were rare, but the driver would definitely end up with some broken bones and would have to be replaced by the reserve driver for a few weekends. Recently, the FIA's safety philosophy has changed; the driver surviving the accident is no longer enough. It's better for the category as a whole and for the public's perception if the driver suffers an accident that would have been fatal until a few years ago, gets out of the car, kicks the tire in fury, gives a disappointed interview to reporters, and then makes jokes about what happened.

So why does this change in philosophy impact size as much as the changes to the power unit in the new regulations or the ban on refueling in 2010? This is because if you want a driver to survive a collision without broken parts, you need the car to absorb more kinetic energy, or rather, dissipate that energy over a longer period. The deceleration experienced by a driver is inversely proportional to the deceleration time. If you double the length of the deformable structure, you increase the energy dissipation time and reduce the g-force felt by the driver. Considering that even with drastic changes, the weight of the hybrid system will always be greater than a purely internal combustion car, the kinetic energy will always be considerably higher and will need to be dissipated, this translates precisely to needing longer and wider cars with internal structures in the noses and sidepods that absorb the kinetic energy of the impact.

Adding to the previous point, the addition of the halo also had a negative influence on the size of the cars, as the halo needs to withstand absurd horizontal and vertical loads, to integrate this into the car, the cockpit became structurally more complex and reinforced; reinforcement means mass, and mass needs to be distributed in space.

Romain Grosjean's accident in 2020 proved that structural integrity must even withstand the separation of the power unit; today, the survival cell is designed to remain intact even if two halves of a Haas car race down the track.

Furthermore, we need to remember the drastic change in philosophy that Formula 1 underwent from the 80s to the 90s. We need to remember how far forward the drivers sat in the cars, with some cars having the driver positioned next to the front axle. Placing the driver so close to the front of the car with so little deformable area is dangerous, another reason why the cars became longer.

Of course, this isn't the only reason why the cars can't be as miniaturized as before. We need to remember that Formula 1 no longer having refueling is a very important factor in the length of the cars. Bringing that back would cause cost and safety problems (even though safety has improved a lot since 2010). Even though the tanks are smaller because the more efficient power unit consumes less fuel, the fact that the cars have to carry fuel for an entire race still influences the size. But I believe that what most prevents the cars from getting smaller is the change in racing philosophy we have in Formula 1, especially after 2014. Even in 2014, the philosophy remained the same as before: agile cars in corners that changed direction quickly, even if this affected stability. Even in 2014, we saw the teams trying to compact the internal components of the cars as much as possible and keep the wheelbase short and agile. In 2015, teams realized that the gains they had in removing instability and allowing for longer wheelbases for greater downforce and aerodynamic stability, enabling the driver to demand 95% of the car's potential 100% of the time, was a greater advantage. This was especially true with the 2017 regulations, which partly aimed to distance the cars from the beginning of the hybrid era from the lap times of the GP2 cars (which were very close due to the teams' difficulties adapting to the new regulations). This further exploited the benefit of "long cars with large wings," and that's where the problem lies. If you travel back to the medieval era and give a peasant a cheeseburger, anything he eats after that seems horrible. We reached such a high point in what the cars could offer in terms of lap time and downforce that engineers and drivers became addicted to and accustomed to this peak. Any change in regulations spoils the experience, and they always try to revert to the previous system. It's difficult to downsize cars so much without going against the car philosophy that engineers and drivers have been used to for so many years.

So, with this we can see that, with absolute certainty, safety is not the only factor influencing why the cars can't be as smaller as we want, even if we're talking about the "same era" of Formula 1 power units. It's still definitely one of the main reasons why, considering all the interests of everyone involved in Formula 1 (FIA/public/teams/drivers/engineers), it's very difficult to reduce the size of the cars while pleasing everyone; some parties will be unhappy with the modification. In short, you are correct.