1. You have automatic scaling on your printer / computer on. This scales the image to fit the A4 paper which could result in altered sizes.

2. Your printer uses A4 paper. However, most (except high-end photo printers) have a border around the paper which it cannot print on. This is a hardware limitation, where printers are unable to print the edge of the paper.

If you want to solve your problem, you can buy a borderless printer (one of the photo printer models) to get what you require.

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