Can the length of disc brake hose influence braking power?
This sounds like an incomplete bleed. If you hang your bike vertically, try pulling the rear brake lever after it has sat overnight (front wheel up). if it feels extra bad that confirms it. Or if you notice that the brake feels better after a few 'pumps'. 

That said, I had noticed with my SLX brakes that the front and rear had a different 'feel' possibly. Not that they acted differently at all, but felt different through the stroke of the lever. I thought it was possible it was the factory bleed, which wouldn't be the case for you, but perhaps there is something to it. Or possibly a difference in friction in the Pistons in the caliper or lever somehow. 

On my Jamis dragon actually (separate M675 from above), I think the front brake ended up with too much oil from the factory bleed on the front. I couldn't get the pads open enough to not rub the disc. I put a bleed block in the caliper and opened the bleed screw on the lever and that pressure alone pushed a bunch of oil out. I closed it up and everything worked as it should have. Possibly something similar has happened in your case, but too little oil? (this could happen if the Pistons were too far retracted in caliper/lever not fully open during bleed)

side notes: Of course the front lever is going to provide more stopping power as it has greater mechanical advantage. Also some bikes have a larger front rotor. And never rule out quality control.