sim card sizes. Why start with the big one
The miniaturization of electronics is a process and takes time to refine a design, as well as manufacturing processes that physically create the electronic device, down to their most efficient state.

At the time of its inception, the SIM card used an internal chip that was a specific size.  As the design of the chip was refined, the SIM card no longer needed to be so large, hence the miniaturization of your overall SIM card.

This is the inherent progression of designing chip electronics