I have been using SwiftKey for a few years now and have a large dictionary. Still, my data size is some 17MB. Yours at 845MB is enormous, and this is likely due to some bug/error inside the app.
Here's what I can recommend:
Open SwiftKey and go to Account > Backup & Sync.
Here, ensure that you are backing up your SwiftKey account to Google account, Microsoft or other available options. This will ensure that your dictionary and learned words are synced to the server, and hence the learned words are connected to your account.
At this point, you should be safe when you delete the SwiftKey data and/or re-install the app. After the initial setup, you will be asked to log in to an account, and if you log in using the same account, your dictionary will return. This time, with a much smaller size (expected).
Check that your backup exists and your dictionary is synced:
If you want to confirm that your dictionary is backed up before deleting the data of SwiftKey, you can install SwiftKey Beta and then login using the same account. Now start typing and see if you are getting the predictions as expected. If you do, all is good, and you can delete both apps, re-install stable SwiftKey and have fun.