The file size is changed during the import because with the settings you have selected in your screenshot the shortcuts for the imported items are added to the PST file first of all... and then at the end, when ALL of the items are archived in the PST, the shortcuts are moved to the mailbox you specified.
Yes 10 Gb will take some time to import, but it's a background task right, so if it takes all day, that's fine? You can test how long it will take by importing a 1 Gb test PST, and timing it... then multiply by 10 (of course).
There is no real limit on the size that you can import. Enterprise Vault will just keep on ploughing through the PST file.
One thing I wouldn't do.. but it is entirely up to you (governed probably by the CONTEXT of the data in the PST file) is that I wouldn't import it to the root folder, merged. This'll overly all the things in the PST over the mailbox. I would "normally" import to a folder called (say) ImportedPST
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