The only thing I can think of regarding the design and decision to put a 23 gallon tank on the etourque is a slight uptick in estimated fuel economy to drive a sale for those that find that info important.
According to the specifications section on the Ram website, quad cabs come standard with 23-gallon tanks, while crew cabs come standard with 26-gallon tanks.
To your point, if I attempt to build a Laramie 4x4 with HEMI eTorque online, I see the "standard" tank shows as 23 gallons. There is a $0 option for the 26-gallon tank, but that option requires the Laramie Level 2 package ($4,895). I'm guessing you don't have the Level 2 package.
My dealer is taking care of me though, they said they will install a 26 gallon tank for me at their expense. They said FCA won't pay for it.