iPhone Mini with its H/W & S/W integration barely manages 4Hrs of SoT. An 'Android Mini' phone with its mini batteries, how can it match upto iPhone Mini? And mind you, low sale of iPhone Mini is also due to the 'battery/range anxiety' that its users have.
Upon that any Mini form factor needs to be even less thinner, as visual perception of thickness is inversely proportional to a form's face/back surface area. So for this mini phone to be reasonably attractive (not chunky) it needs to have a very slim profile; which translate to small battery.