2014.12.15 17:49 "[Tiff] Unnecessary bigger JPEG-compressed TIFF", by Even Rouault

2014.12.21 02:24 "Re: [Tiff] Unnecessary bigger JPEG-compressed TIFF", by Tom Lane

And from my testing with old imagemagick it seems that gm convert doesn't produce a YCbCr JPEG-compressed TIFF.

Hmmm...

Does YCbCr make that big of a difference?

Possibly. What were you doing instead? RGB output would be substantially larger than YCbCr, possibly approaching 3x larger. (That still isn't enough to explain the reported size difference though.)

                        regards, tom lane