Highest I have seen is about 120,000 pending txs from nodes that have restrictions.
In addition, during times when the btc price is increasing the mempool increases.
Whether that increase is due to the price or the price is due to the mempool size is debatable.
Do you happen to remember when that was? It's interesting to figure out whether or not it was a peak in usage or a spam attack that resulted in such (what I consider to be) an insanely high number. If I look at the current situation, then I think it's purely the result of a peak in the usage due to the price. The majority of the people don't pay a single bit of attention on how the network is doing when transacting.
Yes, the largest was in September 2015 and it was actually around 180,000+ pending.
I assume it was mostly spam attacks at the time, I do not have the data now to determine otherwise.
Could have been no fee attacks which took 2-3 days to be dropped.
That was the time (2015) when I began to see unusual random spikes in the mempool.
Some seemed natural, others like the 180,000+ txs did not.
Edit: Here is a picture of mempool data graphed from Statoshi
Find more data at Statoshi's Node -
http://statoshi.info/dashboard/db/transactionsSet the date range on the top right.
