I expect it is because you have left the emails on your ISP's server, so they are all downloaded again to your new email program.
When you set up the receiving part of your email program there's an option to "Leave emails on the server" or delete them when they have been downloaded. If you never delete them from the server, eventually it'll fill up, and your ISP may start deleting the oldest ones [check their policy].
My desktop (repository for all emails) fetches emails from my ISP, then deletes them from my ISP's server.
My laptop leaves messages on the server, so they can be fetched by my desktop.