Why is Gimp JPG so large?

[rich2005]

Quote:...snip...Would really like it to take up the same, smaller amount of space as other documents I have of the same dimensions.

Well AFAIK you can get ImageMagick (IM) for MacOS and one command is compress to a (roughly) specific file size. Example: Quite a colorful jpeg scaled up in GIMP to your 10800x7200 = about 13 MB jpeg filesize, exported as 90 quality with chroma halved, no thumbnail, no colour profile. 

To recompress, The IM command is

Code:

convert a.jpg  -define jpeg:extent=2000kb newfile.jpg

Still using IM, the identify command, comparing the properties.

Image: a.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Class: DirectClass
  Geometry: 10800x7200+0+0
  Resolution: 300x300
  Compression: JPEG
  Quality: 90
  Filesize: 12.8521MiB
-------------  
  Image: newfile.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Class: DirectClass
  Geometry: 10800x7200+0+0
  Resolution: 300x300
  Print size: 36x24
  Compression: JPEG
  Quality: 10
  Filesize: 1882910B
  
Look at the estimated quality 10. I would be very wary compressing large images too much.