WhatsApp:
- Doesn't have a website.
- Because there's no website there's no need to localise everything, just the app.
- Doesn't let you embed messages in web pages.
- You can't 'retweet' messages.
- Does WhatsApp moderate the content of messages? No, so no need for a team to do that.
- You can't follow people's every message in every chat, because they're generally 1:1 conversations. Twitter does this as Direct Messages.
Pretty much, Twitter does WhatsApp in the DMs.
If WhatsApp doesn't need to do some of these features it wouldn't need to employ people to develop and maintain them.
WhatsApp and Twitter are completely different.
Whatsapp does have a web client FWIW
Twitter will need more staff largely for content moderation but at half the amount it has now it will have seventy times the amount Whatsapp had.