Do you use layered Tifs?

If I may just add, the embedded "flattened" preview that is saved by Photoshop into a TIFF - or into a PSD for that matter when the "include compatibility preview" option is selected - was described above as a JPG. I believe it may be lossless compressed, but certainly it is not a compromise in the same way that the embedded preview of (say) a Raw file is.

That is why the filesize is so large - but it is also what lets programs such as Lightroom, Qimage, and others beneficially USE these complex PS-edited files in the first place. While PSD can be saved without this extra stored "layer", and so be more compact, that is not realistic for those using partner programs.

IOW, this is not some second-rate thumbnail just for browsing. It is a full exact representation of the layer stack that you view while in Photoshop itself. When there is only one (background) layer, such a compatibility preview is not made since the Background itself can serve this function.

[BTW a far greater filesize impact is involved in the choice of 16-bit vs 8-bit, also in the decision to use, or not use, Smart Object function when processing from a Raw file. Most importantly, in the decision to convert to a file in the first place, rather than working parametrically from the Raw wherever that is possible.]

RP