Hi ShaggyShepherd,
Thanks for reaching out using Apple Support Communities. Here to help.
We understand you are seeing a decrease in image quality when you upload photos or videos via WhatsApp.
Do you use iCloud Photos on your iPhone? If so, you may want to make sure 'Download and Keep Originals' is enabled. This will ensure that a full-resolution copy of the image stays on the iPhone and is available to other apps that want access to your Photos. This article discusses this feature:
Download iCloud photos and videos
As stated in the article, to check this feature,
"Go to Settings > [your name].
Tap iCloud, then tap Photos.
Select Download and Keep Originals."
It appears WhatsApp does have file size restrictions, so this may be something else to look at. Certain camera modes such as HDR and 4K video may be large files and are perhaps being compressed by WhatsApp. You can manage these settings in Settings > Camera. We have attached a potentially relevant support article from WhatsApp below.
Troubleshoot blurry photos- WhatsApp
Hope this helps! Take care!