Note: 100 kB per 30 second share is a bit of an oversimplification of how the mainnet code works. In reality, it's sometimes more than 100 kB (when transactions can be resurrected from a previous block round), and often less than 100 kB (when different nodes are using different transaction sets, or when the transactions in a template change, or when transactions are large). I have looked closely at the shares mined for two block on mainnet. In the first instance, it took around 20 shares before it got up to 1 MB instead of 10 shares. In the second instance, it hadn't gotten to 1 MB even after 16 shares. In both, there were several shares that were actually smaller than the previous share.