I've the same issue too, since last iOS update (15.4.1.) and after many searches I've found the cause of the problem: vocal messages and especially the STICKERS!
WhatsUp inbuild Manage Storage doesn't include the vocal messages and the stickers size.
Example:
in the WhatsUp inbuild Manage Storage, it show me something like 120 MB of chat (include media, pictures and videos).
in the iPhone storage (related to WhatsApp), it show me something like 2.5 GB and I didn't realize where they came from...
So, in the WhatsUp inbuild Manage Storage, when I check the chat size with my girlfriend it show me 30 MB, but when try to export the single chat (the one with my girlfriend with TONS OF STICKERS :)) the .zip file created by whatsapp was 2 GB.
I exported the zip in Google Drive, downloaded/extract the zip file in my computer, and there was tons of STIKERS in .webp file format, for something like 1.5 GB (I'm not joking...).
I wrote to WhatsApp customer center (and wrote a review in the app store) in order to ask them to improve the function to check/delete (as well as pictures and videos) also the vocal messages and stickers in oneshot mode for a single chat, 'cause it's frustrating scroll a 3-year chat and delete one by one every single stickers.
Hope it helps and sorry 4 my english.
Fabrizio