SQL queries tend to perform differently with more or less data, or when information about the object changes. An object that was originally 2MB may now be 2GB; more of the columns of a table might be populated after the initial load, which didn’t have complete information. The information about the database objects and data is used by the database servers to figure out indexes and execution plans for queries.
In both SQL Server and Oracle, statistics are updated by default. In SQL Server, the AUTO_UPDATE_STATISTICS database option, when turned on, will update the statistics when they become stale. You can also run updates manually, using sp_updatestats or UPDATE STATISTICS.
In Oracle, the parameter STATISTICS_LEVEL set to