12 gigabytes for a 2000 pixel square image? I can't imagine. However, the dimensions are not so much the problem...it's what's in the layers that he's made that make up that 2000 pixel square image. It's the complexity of the data that enlarges the file. For instance, if I fill a 2000 by 2000 pixel layer with a single color (every pixel the same) that will barely make a dent in the file size because Photoshop will do some shorthand and just say to itself "This color...4,000,000 times". But if you fill that layer with intricate detail, or gradients of color, or clouds, or effects that influence other layers, that's a whole lot of calculations that have to be done to each pixel in the layer and the file size will get really big really fast.
But 12 gigabytes? We'd have to see what he's done in that file to figure out why it's so big.
Just to give you an example, my Planet Eben political map has approximately 1300 layers. Most of them are text labels and there aren't many color gradients. The dimensions are 8200 by 4100 pixels and when the file is open it's 2 gigabytes. Saved and compressed it's 346 megabytes.