Adjust photograph size on Photos/Preview

I have a photograph in the Photos app in the 4:3 aspect ratio. I would like take a 12"x9" print. I could not find a way to size the photograph in the Photos app (I am aware that aspect ratios can be changed). Therefore, I exported the photograph in full size with maximum quality as a JPEG file (3.1 MB).

The JPEG file was then opened in Preview because it has an adjust size function under Tools. In adjust size function, the original size is seen as 38.6"x28.94". Upon adjusting the size from original to 12"x9" the file size reduces to 1.4 MB (Resample image-unchecked) or 187 KB (Resample image-checked). Can someone explain what is meant by resample image and the drastic reduction in file size? I was not able to find any explanation in the Help menu item.