Hi there,
I would imagine that when you set up your new iPod, the mail app tried to update itself, and downloaded all the emails from your email account.
Generally, the mail app will clear old emails off your phone after a certain amount of time, but they still remain on the server which runs your email address. In other words they aren't gone forever... you'll just have to search for them, or go to your email provider's website and log in to see them.
Your new iPod may have the setting turned on to download all emails. I believe there is a setting to help this.
-Go to your settings, and scroll down to "Passwords & Accounts"
-From there you should see a list of your account(s), which would include your email account.
-Tap on the account you use for email
-Tap "Mail Days to Sync"
-Tap on the desired length of tome you wish for messages to stay on your device. I personally have it set for 1 week.
I would imagine on that setting it may be set to "No Limit" for whatever reason, which would likely cause the large amount of emails on your device.
It may take a bit of time for those emails to be processed once you make that setting change, so just know they might not disappear immediately.
I hope this helps!