This is a difficult question to answer definitively as there are many ways to measure "size." Some people might consider the company with the most employees to be the biggest, while others might consider the company with the most revenue to be the biggest.
One popular metric for measuring the size of a fintech company is funding raised. By this measure, the largest fintech company in the world is Ant Financial, which is the parent company of Alibaba's Alipay. Ant Financial has raised over $34 billion in funding, according to Crunchbase.
However, it's worth noting that Ant Financial is not solely a fintech company; it also operates in other industries such as e-commerce and artificial intelligence. So, if you're looking for the largest pure-play fintech company, that would be a different organization.
To conclude, there is no one "biggest" fintech company in the world; it depends on how you measure it.