Go to setting, iphone storage and check which app using the most memory.

photo/video and whatsapp can used alot of memory especially if you have sync to ur account. And you said you're using 64gb one, IOS normally taken up 2-5gb. Plus minus the IOS itself need some memory to operate the phone.

Others included any sort of data, not only whatsapp. Like you browsing/ using siri/ or any other thing which save alot of cache into ur phone.

Best is you switch off all the auto-backup & auto sync option, and see how it goes.

hmm, really no idea which apps consume all your data.
Btw, go to your iphone settings, not whatsapp setting

Settings --> General --> iPhone Storage, see which app keep increasing in size.

Best is restore as new, since your whatsapp is sync to iCloud. Can be restore later in the app.

Try disable all sync process after restore as new, den install whatsapp only and restore it. And see if the culprit is whatsapp, try disable auto download in whatsapp if possible. Especially if you got many groups

If you got alot of media (Photo/ Video), actually they will have a cache too in IOS. Which you can browse/ load it faster the next time when you looking for it. It is like your web browser, it will save a cache so the next time you browse back the same page will be faster.

So there's nothing you can do about it, perhaps you need to prioritize which media is important and which can be clear in your whatsapp. Otherwise I would suggest you to upgrade your phone to bigger memory or nowadays you can transfer the media to an USB drive for IOS too