On Nov. 15, Bitcoin’s (BTC) blockchain mempool was at its highest level since January last year, according to figures from blockchain data website Blockchain.com.

On Nov. 15, Bitcoin’s mempool size reached over 90 megabytes (MB), a value not seen since January 2018. The Bitcoin mempool holds all the unconfirmed transactions that are waiting to be validated by miners.

One-year chart of Bitcoin mempool size in bytes. Source: Blockchain.com

An apparent anomaly

In most cases, the bigger the mempool, the more transactions are waiting to be confirmed by miners, but in this case, the situation is quite different. At its peak, the number of unconfirmed transactions sitting in Bitcoin’s mempool on Nov. 15 was a little over 20,000.

The number was higher — if only by a couple hundred of transactions — only two days before, with a mempool of about 12.6MB at its highest. All of this data seemingly indicates that last weekend, the average size of transactions was significantly bigger than usual.

Bigger than usual transactions generally involve many inputs or outputs, or simply store data on the blockchain. Alex Saunders, the CEO and founder of Australian cryptocurrency news outlet NuggetsNewsAU, suggested in a tweet on Nov. 17 that the cause of the large size of the mempool was the activity of cryptocurrency exchange Binance.

Saunders said that the mempool was filled by Binance moving small quantities of Bitcoin or Omni-based Tether (USDT) from many addresses with the lowest possible fees, which would result in the transactions being held in the mempool for more time.

At the end of 2017, Bitcoin saw its mempool reach 120MB and transaction fees cost as much as $16. At the time, Bitcoin analyst and researcher Nic Carter suggested that the mempool showed suspicious behavior because the network was flooded by a great number of transactions sent with the lowest possible fees.