I have this problem using an iPhone 6 or iPhone 5s with either of two sets of wireless headphones - Plantronics Backbeat Pro and Plantronics Backbeat Sense.
The volume over bluetooth is much higher for the same number of volume bars compared to wired, to the point where I'm severely lacking accuracy at the low volume end. 1 bar is muted, 2 bars is too loud for when listening in quiet environments. When wired the volume range is perfect.
Volume cap doesn't fix this. Analog control of the volume in the popup control center is not enough.
Within iOS, EQ is off, volume limit is on (EU limit) Sound check off.
How can I fix this?
I've tried various manoeuvres with pairing, un-pairing, updating firmware, pairing to different devices, and back again. I don't have a non-apple device to compare with, but the same behaviour occurs with two different iPhones and the two different headsets as described. Plantronics support struggled to see how this was a problem, but acknowledged the discrepancy between wired and wireless volume ranges. I had a similar problem with a pair of V-moda crossfade wireless headphones.
I'm not sure if this is a apple or bluetooth issue (i.e. 'profile' problem) or a hardware issue i.e. the gain is just too high.
I've found that this issue is head-phone specific, perhaps brand-specific or in all likelihood, bluetooth implementation specific. Of the few others I've since tried I found that Bose wireless headphones to not have this issue. However, other problems with distortion sometimes occurred instead!