CS5 - Photo Saving and File Size

recently i've noticed, when uploading photo's web site file size rather small.  know cropping, processing, , saving jpeg results in file size reduction.

 

so ... did test.  took 1 photo shot in raw format (the file size 21 mp).  took photoshop cs5 , saved jpeg ... , file size went 9.9 mp.  found questionable reduction.

 

my image size details of test photo is:

 

pixel dimenstions:

5616 px x 3744 px

 

document size:

18.72 x 12.48 , 300 px resolution

 

all of following checked:

scale style

constrain proportions

resize image

 

i have selected:

 

bicubic (best smooth gradients)

 

i saved photo @ highest quality setting of 12

 

so question:  test reduction normal or doing wrong "save as" settings

there no set percentage of compression, depends on image content.  if there layers jpeg not save in reduce size.  web options strip out metadata.    thinking or little compression?

 

i know there can significant differences in saving @ image quality 9 or 10 , @ 12.  try , see get.  correct compression depends on end use.  web use maybe need 5.

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