I just spent a lot of time testing this last night, was going to write up a post but may as well add it here. I have more details (don't have my notes with me) but my testing lines up with OP's results.
I tried blues, purples, exotics, all got the same amount of XP.
Also tried equipping some fully leveled items, didn't change how much XP was applied to non-leveled items.
I did my testing by loading destinystatus.com, applying some XP, then reloading destinystatus.com
Class item boost means equipping a class item that has "increased rate for hand cannon/etc"
Weapon, Armor (including Ghost shell), Subclass/Light Level Bar
2500 XP Bounty
Weapon: 525 XP
Armor: 500 XP
Subclass: 2500 XP
3000 XP Bounty
Weapon: 600 XP
Armor: ?
Subclass: 3000 XP