Why are exported TIF files so large?

I was asking this same question recently when I noticed that a 27MB image (from an Nikon D610 6016x4016) ballooned to 106MB when I chose to Edit in Photoshop from Lightroom. The replies here are the closest that I came to an explanation, but they really don't answer the above question. (I also have the same problem with my Sony A7 RIII.)

I noticed the problem when I started to run out of storage on my SSD. I am compositing images, and one of my .tif images grew with Layers to over 1GB. I expect that Exporting to .tif produces the same problem.

Looking into the image file tags with ExifTool and MATLAB and comparing them with the Adobe TIFF 6.0 and DNG 1.5 specifications and the ISO TIFF/EP specifications, my .tif files match the TIFF 6.0 spec from 1992: There is a short header, 106MB of pixel data, and a series of TIFF tags in a small Image File Directory. The 106MB of data appears to be a compressed version of 5824 cols x 3888 rows x 3 colors/pixel x 2 bytes/color = 135MB. The ZIP compression of runs of RGBRGBRGB data is not that efficient (135MB --> 106MB) because while a run of RRR or GGG or BBB are usually similar in values, the R, G, and B values in most any pixel are not. Camera RAW and DNG files separate the R, the two G, and the B values measured by the Color Filter Arrays and ZIP compress (Deflate) the "channels" separately and more effectively and efficiently. Camera RAW and DNG files use the more recent TIFF/EP specification (from 2013 and 2019) with its support for CFA image data. (To be precise, the .tif files created by Lr and ACR are created under the Profile 1 of TIFF/EP, which is compatible with TIFF 6.0 but includes ZIP compression. Camera RAW and .dng files are created under the more advanced Profile 2 of TIFF/EP.)

Lr and ACR (and GIMP) read and write Camera RAW and DNG image files. They can "adjust" the individual pixels, but they cannot replace the pixels the way that Ps and other image editors can. PS does not save back to Camera RAW or DNG formats, but you could try saving as a PSD file for effective compression. (I have not tried this.) Ps or other image editors, viewers, and printers, however, require images formated in other ways. A TIFF 6.0 (.tif) file is perhaps the most common pixel (raster) image format.

So, we're stuck creating large TIFF files from small Camera RAW or DNG image files when we want to use them in unspecified image editors and applications. You can Flatten your images to remove the layers (each Layer being the same size as the Background Layer (image)). You can also try to stay within Lr or ACR saving as DNG (as I am) and encourage your destination to use these Adobe products (or GIMP). Finally, you could reduce the size and perhaps the apparent quality of your images by saving to JPEG with Quality at 100 allowing for any size. (Each Save throws away some of the pixel information.)

BTW, TIFF/EP (Camera RAW and DNG) has a place in the file structure for XMP (editing commands). TIFF 6.0 can be extended to hold XMP as a Tag. Cmd-/Crtl-S updates the XMP information in the open image file. I expect that Ps/ACR and Lr read either the included or the accompanying XMP data when they open either .tif or .dng files (as long as you have used Smart Objects in Ps for non-destructive edits, otherwise, it makes no sense to track the changes that have permanently changed the original file).

>> "I guess I wish there was a single format that stored the RAW files AND the lightroom edits (not in catalogue) to easily share while collaborating."

Depends what you mean by "collaborating.". If the recipient will be revising or adding to your edits using Adobe software, then DNG. Otherwise, Tiff. And if your version is the final version and in Adobe RGB or sRGB space, it can be 8 bit/channel (half the file size).

Thanks!  I guess I wish there was a single format that stored the RAW files AND the lightroom edits (not in catalogue) to easily share while collaborating. 

The raw file is grayscale - one channel - at 14 bit depth. A TIFF is RGB - three channels - at 16 bit depth.

The TIFF isn't "large", it's what a 7952 x 5304 pixel RGB image weighs in at. That's the native uncompressed size.

The jpeg format goes the other way. It aggressively compresses the file by chunking up data and throwing away a lot of information in the process. The net result can be 2-5% of the TIFF file size - but the original quality is irretrievably lost and for this reason jpeg should never be used as a working format. The file deteriorates with every resave and eventually disintegrates completely.