Really no secret there! If you look at the structural features of the SSD drive, then everything becomes clear.
The volume of SSD disk is formed by the sets of memory chips. Each memory chip is connected with the controller.
Typically, SSD drives of the same model equipped with the same memory chips. I.e., SSD 256 GB requires 8 memory chips of 256 Giga-bit, for SSD of 512 GB respectively requires 16 memory chips.
Thus, a 512 GB SSD will have twice as many flash chips of 256 Giga bits. Since each chip will be individually connected to the controller, the controller can record/read 2 times more information per unit of time, potentially increasing at 2 times the speed of the SSD drive.
So, everything is very simply explained.