Using the following two commands yields completely different file sizes:
gdal_translate test.tif -of GTiff -co "COMPRESS=JPEG" -co "JPEG_QUALITY=100" output.tif
gdal_translate test.tif -of JPEG -co "QUALITY=100" -co "WORLDFILE=YES" output.jpg.
(I Wrote the commands from my memory, not sure that the syntax is correct)
The jpg file (second command) is 2 times smaller than the compressed tif file. Why?