That’s following exactly what Arduino-ESP32 made by Espressif does:
Note that 512K describes the total SRAM, but it’s split in DRAM (data-bus), IRAM (instruction-bus), RTC Fast memory, RTC slow memory, etc., so not equally usuable. And there’s also the flash cache playing a part in it. You can see the exact memory partitionning done at the linker file.