Check the metadata. That could be part of the issue. One other space hog can be items "off" the edge of your canvas. Do you have a bunch of elements along edges of your canvas, with parts "off" the canvas? That can also create problems. But even then...that's a huge file size! I'm curious how many layers it is? (I'm also curious how big that TIFF will be when you save as a 12x12 300dpi jpg?) My biggest file, which has a LOT of layers and a bit of that "layers off the edge of the canvas" problem is 271MB.
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