So check, if you have to update your WhatsApp version in the App Store.
The new feature called Storage Usage enables you to individually remove storage-consuming files from the chat history of each contact. You can keep the text, but you can get rid of the e.g. videos.
Then you can also find the option Storage Usage. This level will reveal to you a list of all chats on your iPhone. It starts with the chat, that has saved the highest amount of data. Open any chat you like by tapping it.
These detailed screen will break down the total of data size into text, images, GIFs, videos, voice messages, documents, contacts and locations. You will see the respective data size and amount of files next to it.
This way you can pick the most space-consuming files e.g. videos and keep all other files in your chat history (screenshot).