I had a frustrating evening trying to work out why my ESP32 CAm was not recognising the PSRAM and only taking pictures at 800 pixels. I was taking 1600 pixel photos a couple of days ago, but not now. I tried lots of things (3 h worth!) and then I saw the board was set at ESP32 Dev (which has PSRAM disabled) in the Aduino IDE, not AI Thinker CAM in the Arduino IDE.
I changed it to AI Thinker CAM and it works. I thought I post this to save other a few hours of frustration.