Personal Folders files (PST files) are Outlook's repositories for email, contacts, calendar information, and much more.
When you delete an item in Outlook, it is moved to the Deleted Items folder, without freeing any space in your PST.
After you empty the Deleted Items folder, the size of PST will not change; the deletions will leave "white spaces" within it, as a result of the previous fragmentation that it runs (because it is a mini-database).
So, when you save items in your PST, its size actually increases. When you remove items, it does not decrease, but keeps the size thanks to those white spaces, unless you compact the file.
Our process puts all existing items in an order that ignores the white spaces of the original PST file, so the size of the migrated data may be different, usually smaller, although it will contain all your data.