515 KB is the average block size in the network of bitcoin in recent days. As Armin van Bitcoin at Twitter noteed, this is due to general technical improvements in the protocol.
For example, Armin van Bitcoin is convinced that such numbers became possible due to batching of transactions in the work of a number of large bitcoins, growing acceptance of the SegWit protocol, as well as increasingly frequent off-line transactions (for example, in the Lightning Network) .
Armin van Bitcoin at Twitter
Activated in the network bitcoin in August 2017 protocol SegWit makes it possible to increase the block to 4 MB, thus refuting the allegations of the apologists for various forks of the first crypto currency that only their implementation can solve the problem of scaling.