An interesting (to me, anyway) thing happened last week. I went away for the weekend, and it was the first time is taken my iPad Mini away with me. Wanting to travel light, I took both USB cables (for the iPad and the iPhone) but just the mains charger plug for the iPad. Turns out, it's geared to a waaay lower ampage than the iPhone one. When I plugged my phone into it, I assumed something was broken - the ampage is so low that the phone doesn't register as being plugged in at all. It did charge, but took about 9 hours to get up to 100%.
I'd noticed the iPad takes longer to charge than the phone does, but then the batteries are bigger, so I didn't think much about it until now. In future, I'll have to remember to take both plugs (and remember which is which, since I have no idea what might happen if I plugged the iPad into the iPhone charger).
_________________________ If it's brokenless, don't suffix it...
What's odd in your case is that the iPhone draws less power than the iPad will when charging so the iPad charger should have been more than enough for the iPhone.