Its most likely just poor image compression from their cameras or scanners or whatever the drivers are using. I once worked on an app on phones that used libjp

Its most likely just poor image compression from their cameras or scanners or whatever the drivers are using. I once worked on an app on phones that used libjpeg because te built-in 'hardware assisted' jpeg encoder was so bad. The typical photo was 5MB and simply reencoding it with libjpeg defaults resulted in under 1MB. A simple bit of command-line magic can typically shrink those images massively before you email them onward.