WeChat has always been part of Tencent. It was piggybacking on the infra of what was already one of the biggest tech companies in the world. That's like comparing the number of Gmail employees to Twitter, sure it serves lots of users, but it doesn't have to worry about so much of what a standalone company has to deal with
They weren't making money with those 1000 employees, either. Revenue 10X'd since then (along with cost).
WhatsApp/YouTube were in the same boat when purchased. Good at getting active users but monetizing takes a lot of employees and they hadn't tackled that yet when acquired
WeChat has always been part of Tencent. It was piggybacking on the infra of what was already one of the biggest tech companies in the world. That's like comparing the number of Gmail employees to Twitter, sure it serves lots of users, but it doesn't have to worry about so much of what a standalone company has to deal with