Pictures transfered from PC to iPhone 7 there is pixel reduction

I download pictures from my PC to my iPhone 7 with iTunes. I do not use iCloud or WiFi. I use a USB connector from my iPhone to the USB port on my computer that's it. I recently had a picture on my PC with a pixel size of 4000x4000 when I downloaded it to my phone the pixel size was reduced to 2048x2048. Is there some kind of setting I don't know about that reduces the pixel? How can I correct this problem?