Have you tried different throttles? Did you get any whatsapp account(s) banned at higher speed?
It must be either message size, or WhatsApp using a separate host name for attachments.
Much easier to implement than encrypted package inspection.
How does Whatsapp choose to ban people? Total messages sent? Then increasing the message size doesn't stop you from being banned, it just makes it take longer before you're banned. Messages/second? Then avoiding the ban by a message size restriction seems like a very clumsy way of doing it; it would be much more straightforward to avoid the ban by limiting the messages/second.