File sizes out of LR: D750 vs D7200

I today did a 'corporate portraits' photo shoot using the D750 with the 16-35 f4 and the D7200 with the 16-80 f2.8/4.

Both were set to 14-bit raw with lossless compression.

The client requested all images at full resolution and uncropped, so 6000 x 4000 from both bodies.

Once I'd processed the raw files, all at 6000 x 4000, I noticed the JPGs taken with the D750 were 8-9Mb in size, while those from the D7200 were 12-16Mb in size.

Any ideas why the difference? I could understand if the D750 files were larger - this seems counter-intuitive.

Were the RAW files the same size?

If so the JPEG files were likely saved with different compression factors. All JPEG files are compressed.

Raw files out of the D7200 were about 30Mb, of the D750 about 32Mb, so that looks right. I used the same settings for export for all the images from both bodies -which adds to the mystery.

All I have after that is the actual photos themselves. I do not know anything of what backdrops you used. But in a general sense, high-contrast slightly busy patterns will compress better than a uniform color gradient will. Actually a gradient fill background like what is popular in many slide templates for MS Powerpoint will choke the CPU of a color printer as it has work nearly each pixel individually during rasterization. I often see a service call for slow printing because of that in my job.

So if you used the classic 1-color backdrop lit to give a gradient of light behind the subject like I often see how corporate portraits done - we could have a suspect.

Can you verify that the "color spaces" are the same?

I have no clue but wonder if pixel density might play a factor. After just comparing NEF's from a D7200 vs a D750, the D750 file sizes are 6016x4016 while the D7200 sizes are 6000x4000. But that doesn't offer any insight. One thing I noticed is that not all NEF's from either camera have the exact same MP size. Some are 1MP or 2MP greater in size. So I'm guessing part of it has to do with how much content (such as contrast between lights and darks) was captured as well as how much processing was done to the final images.

Maybe @BackdoorArts can offer some insight?

Folks have mentioned the probables here - different quality, compression, and/or ppi values when saving (something that is said to have been ruled out), and color space.

If everything is truly the same on each then the only other thing I can think of is that the content of the images from one camera is consistently different enough that they exhibit more details and colors and therefore are less compressible when saved. A sample of each (RAW & JPEG) might help, though I understand that you may not be able to do that publicly because of the content. If you want to PM me with a link to downloadable versions I'd be happy to look at them and see if I can figure it out before destroying them completely.