@Zooplankton is right. If you print 1200x1500 at 1200 dpi (dots per inch) it would be 1 inch wide when printed at full resolution.
Instead, if planning to print, as mentioned, choose A4 size in the new canvas window and set to 300 or 600 for a truly HQ print.
I work at a minimum on canvases that are 2500 wide just to make digital art look better because there are more pixels available to support the anti-aliasing I want for smooth linework.
Edit: the reason you can see all the pixels when you zoom in is because the canvas size is very small. Think of it like a resolution of videos. 480 p is only 480pixels wide. 1080p is 1080 pixels wide. Because it has more pixels it can handle sharper images cause it has more pixels to create the variety of hue needed to trick our eyes into seeing better clarity.
In-between edit2: When working on art we often need to work on larger canvases to maintain the fidelity of the art. Sizing down keeps more details than sizing up. When sizing up the software has to guess and make new pixels to fill in the spaces of the original work where as when sizing down it just has to remove pixels for a tighter image.
Think of DPI where Dots = Pixels. Cause computer screens are made of tiny squares of light where as printers print in tiny dots of ink, hence 'dots per inch'.