The police box is not the natural shape of the TARDIS, as you might expect. Each Phone box, which resembled a specific type of telephone box launched in the UK in 1929, and was essentially a combination of public telephones and police methods of communicating with the area during patrols. The reason TARDIS has this confusing structure is that it’s simple and incredibly reliable for anyone struggling with complex technology: One incident.
The first episode of “Doctor Who,” “An Unlucky Child,” in 1963, introduced both the First Doctor (William Hartnell) and TARDIS. Unfortunately, the incredible ship just in time broke its chameleon circuit – technology that allowed it to change its appearance to match its surroundings. Since TARDIS happened to be impersonating the police box at the time, it has stuck on this form ever since.
Luckily for the “bigger inside” whole thing, the problem only affects the outside of the device. Otherwise, things can get pretty cramped, and the makeup control room TARDIS tends to get whenever the Doctor regenerates is likely to be significantly less impressive.
https://www.looper.com/743111/the-real-reason-tardis-is-a-police-box-in-doctor-who/ The real reason TARDIS is a cop box in Doctor Who