From there, press the tab that reads Data and storage usage then press Storage usage. You’ll then be presented with a page that lists your contacts and has a file size next to them.
The largest files are positioned at the top, this is most likely the person you’ve sent the most texts, photos and videos to, but not always. Certain contacts will be particularly high if you’ve sent them a ton of photos and videos from last night’s house party for instance, even if you’ve never exchanged that many texts with them before.
The list offered by WhatsApp is quite comprehensive – on each individual page you’ll be able to see a swathe of information ranging to text messages to location shares.