It's not an Apple thing, it's a carrier thing. MMS (green) data sizes are controlled by the carrier. WhatsApp doesn't use MMS and is a closed protocol.

iMessage (blue) sent messages are similarly a closed protocol and unencumbered by the carrier.

The destination carrier determines the maximum allowable size an MMS attachment can be:

    Tier 1 Carriers -- Verizon, T-Mobile, AT&T -- all MMS content up to about 1 MB.  

    Tier 2 Carriers allow MMS content of 600 KB.  

    Tier 3 Carriers allow MMS content of 300 KB.

MMS (“green messages”) are mangled by most US carriers. The carrier will often recompress the media. And phones tend to send it at a lower quality to begin with.

I had never thought of doing that. I just tried, and clicking the + symbol in WhatsApp gives me option to send photos/video, documents, contact, and location.

If I choose documents, I do not see where I can go to select photos/videos in the iPhone camera roll. I presume it may not be accessible that way.

Also, I presume WhatsApp does not want to allow people to send photo/video at full size in their network to reduce bandwidth costs.