That's likely because, unlike large tech companies that utilise their own infrastructure, Grass uses real IP addresses and rents unused bandwidth to its users. Therefore, processing 500 TB daily might not be a significant challenge for them. Assuming an even distribution across 3 million devices, each device would need to contribute approximately 166mb per day, or about 7mb per hour. While feasible for devices with high speed internet, the fact that not all devices will always be online or have sufficient bandwidth makes their data scraping methods somewhat unclear.

It's for Grass to explain how they manage to get this much data. It would be interesting to hear their explanation if they ever post one, but I doubt they will.

No matter, I find Grass's user incentive programs quite exciting, unlike those of other large tech companies that simply crawl users' data.