i grabbed 3 2200 mAh battery chargers from Costco for something like $15. They are only 3/4" x 3/4" x 4" give or take and can recharge a dead phone completely. I think the iPhone 6 is 1800mAh so you can still charge it a bit more a second time. For me, the two charges tend to be enough. Of course, the Costco package had 3 and they were all small and lightweight so I could put 2 or 3 of them in a jersey pocket, saddle bag or goodie bag and be able to ride all day without fear of the phone dieing.

I think one of these chargers weighs equal to or less than a Garmin so if you are going to have a phone as well as the Garmin, there is really no difference in total weight carried as well.

The 3 pack from Costco also had on/off switches for charging your phone and a built in flashlight(not great but OK in a pinch)

It has for me helped to stave off the need for a GPS unit.