That would make sense if they had the same hardware. Different hardware absorbs different current. You cannot deduce anything from battery size.
I'd guesstimate I get at most 3 hours on the 2017 before the battery level gets low enough I have to start planning where I'll next plug in. Not ideal.
I suspect that Apple will be seeing a lot of 2015 MBPs brought in for battery replacements...
physics might have a word with you :)