I was on a boat recently where WhatsApp was free to use, and you had to pay to use the rest of the internet. You could send and receive messages but attempting to send an image, which wasn’t even all that big in size, did not work on the free connection.
It must be either message size, or WhatsApp using a separate host name for attachments.
I would not be surprised if the free WA messaging is implemented by whitelisting the signaling ports and domains (XMPP or similar) which only handle text content and small inline attachments. While larger images are uploaded and fetched out of band (HTTP or similar) with only a URL or reference passing over the signaling channel.
They probably use some deep packet inspection on shore side firewall which blocks audio/video. Quite normal on congested satellite connections. Most “next-generation” firewall providers have predefined signatures for WhatsApp file transfer.
It must be either message size, or WhatsApp using a separate host name for attachments.