These files are created sequentially and contain raw block data, which includes all transactions that have been verified and added to the blockchain. The size of these files typically ranges between 128MB to 134MB. This specific size range is not arbitrary but is the result of extensive testing and consideration by developers to optimize for efficiency and performance.

Efficiency in this context refers to the balance between file size, read/write operations, and synchronization speed with the network. Larger files could potentially reduce the number of read/write operations because more data can be stored in a single file, but they also require more time to download and process, which could slow down the synchronization with the blockchain network. On the other hand, smaller files might be quicker to download and process but would increase the number of read/write operations, potentially reducing overall system performance due to disk I/O limitations.

Tests conducted by developers have aimed to find an optimal file size that ensures a balanced trade-off between these factors. While the specifics of these tests are highly technical, they involve benchmarking the blockchain's performance under different conditions, evaluating the impact of file size on synchronization times, and assessing the system's responsiveness during read/write operations. These experiments are crucial for ensuring that the Bitcoin software remains efficient and scalable, capable of handling an increasing volume of transactions without significant degradation in performance.

It's important to note that the choice of blk*.dat file size can have implications for users running full nodes, as it affects the amount of disk space required and the speed at which new nodes can become fully synchronized with the rest of the network. As the blockchain grows and evolves, developers continue to monitor and adjust these parameters to ensure that the Bitcoin network remains robust and accessible to as wide an audience as possible.