From CreativePro.com –CreativePro Tip of the Week: Reduce File Size in Photoshop by Cropping or Flattening:
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Need to reduce the file size of a Photoshop document? You may be able to significantly reduce the file size by either cropping or flattening. If you have a layered file, notice the document sizes in the Status Bar below the image. The two values correspond roughly (sometimes very roughly) to the size of your image as a flattened file (on the left), and as a layered file (on the right). If you don’t need to preserve the separate layers, flatten the image by choosing either Layer > Flatten Image or Layer > Merge Visible. If you do need to preserve layers in the file, try cropping the image. Select the Crop tool and press Return or Enter. You will see a crop overlay fitting the canvas…”
