Scalability refers to a blockchain network's ability to handle large transactions without experiencing delays or performance issues.  As a blockchain network grows, achieving consensus among all the nodes in the network becomes more challenging, which can lead to scalability issues. On the other hand, if a blockchain network is not scaled correctly, it may be more susceptible to attacks such as a 51% attack, where a single entity or group of entities controls a majority of the network's computing power and can potentially reverse transactions or double-spending.