I also had little yesterday seeing a storage usage of three times the size of my Evernote data, but everything was back to normal today, it was a temporary folder.
Like @LeoW said, I also used the Evernote link replacer plugin to convert my old Evernote links into Joplin links. There have been many converts. There are also quite a few that haven't been, but they must be links that didn't have the right name.
Yes, I used it following comments seen here or on Reddit. This app is awesome! I had started exporting my notebooks one by one from Evernote v10, but after a day and a half I hadn't done half of it. This app did everything in a few dozen minutes or a couple of hours.
In fact there seems to be a trick: I realized this when importing an Evernote stack (evernote-backup saves the stack notebooks in a subfolder of the backup directory): I chose this subfolder via 'import Joplin Import ENEX folder to markdown option, and I noticed that although I did not have the stack recreated, all my notebooks there were imported at once into Joplin.
So, even for my non-stack notebooks, I used this import method in Joplin: Import ENEX folder to markdown option by just choosing my main evernote-backup folder, and Joplin imported all my notebooks at once 
Well, it took a long time: around twenty hours, but it's much less than if I had had to import my notebooks one by one.