According to blockchain.info's mempool size chart, the mempool size has increased considerably since genesis, and some initial research shows that the size does grow with abnormal network conditions (high volume spikes, etc).
At the time of writing, the number of transactions is 285,565, totaling about 228.71450055 BTC that are tied up in fees.
That means about 0.0008009192322 BTC per transaction on average. However, following the rule of 20 satoshis per byte, on average, each transaction would need only 0.00002 BTC in fees - about 40 times less than what is currently tied up in BTC fees.
What's going on here? Will the number of unconfirmed transactions die down over time? Will it increase uncontrollably? Will this ever out-scale the number of miners on the network?
Or am I completely missing something here?