The invoice includes scanned bills of lading we get mailed in from our drivers. An invoice size can range from around 2.5mb to 12mb+. An we usually try to batch multiple invoice per email for their convenience. Things like 9 pin printer and colored copy paper bloat the sizes even with max size reduction enabled.
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.
