Could be a bit of both. Some systems just burn the juice faster and maybe because it tastes better youre vaping more. I noticed when I used the Vivi Nova, I went thru so much more mls of juice a day than with cartos in a tank. I switched to my current system and it tastes even better but is much more juice economical.
I have not used a Protank but Im sure some Protank users will jump in soon and confirm if its that tank that burns more juice or just you enjoying it more than you used before.