Connect the QC35 (you didn't say if you have a i or ii) by wire to the device you are listening to and see if the volume is the same. If they are, try a different device to confirm you may have an issue with the qc35. If they are not, especially if you are connected to something other than a phone, it may not be your QC35 but the other end and the way it is implementing bluetooth (bt). I am not having a volume problem with my QC35ii but I am having a volume problem with my Bose revolve speakers as other are here. The volume is much louder by wire than by bluetooth (even on max on both ends). I am not sure what the next step should be. The 'issue' is not with my iphone 7+ (which works fine in BT volume with my Revolve's), but with my two windows PCs. The problem needs to be fixed on one end or the other or I will have to use wires indefinately which I am not happy about. The idea of getting the BT vendor on my PC (originally Intel and now SMK dongle) to change their drivers is highly unlikely. If I change the other end, that means finding another BT solution that doesn't have a volume problem. On the Bose end, they would have to take ownership and then make a change to the QC35 firmware which they have not yet owned up on.
Your 'issue' on volume may be similar and so changing a Bose product may or may not fix your issue. Another vendor's solution may work or may not as well.