Scalability shows the dependency of performance on factors like the data volume i.e. data stored in a database

The response time of an SQL query depends on many factors. The data volume is one of them. If a query is fast enough under certain testing conditions, it does not mean it will be fast enough in production. That is especially the case in development environments that have only a fraction of the data of the production system.

It is, however, no surprise that the queries get slower when the data volume grows

in short we can say that if we want to improve scalability of a query we have to improve our SQL SERVER performance & capacity management