When planning for memory needs, it is critical to understand how SQL Server Integration Services uses memory. SSIS will allocate memory from the unallocated system memory for each package executed, and surrenders that memory shortly after the package completes its execution. The memory allocated for SSIS package executions runs in the SSIS execution runtime process (ISServerExec.exe, if you are executing the package from the SSIS catalog).
Here’s where the package design has a significant impact on memory use. If a package uses an SSIS data flow, all of the data passing through that data flow is written to memory used by SSIS. For example, consider a package that loads 10 million rows from a flat file to a table. In this case, all 10 million rows will pass through the SSIS memory space during package execution.
Read on as Tim goes into good detail on the topic.