I remember testing experience in 1.10 patch in normal game single player.
I thought the xp would start dropping off from baal runs after lvl 40 or so. But the xp actually went gradually
UP after about lvl 45 and there was no drop in xp till about lvl 60. It was quicker to lvl up in normal than nm
This may well have been a patch bug as I have not tested in real 1.10.
My Ladder chars - conc barb(91) and wind druid(91) - get around 4.5 to 5.5 million xp for a solo player diablo and baal run, depending on boss packs you meet along the way etc.
Druid(92) now gets about 3 to 4 mill per diablo & baal run.
They both put on xp at the same rate up to lvl 91 and I never noticed any strange xp changes.
I know the xp in foothills and beyond doesn't really kick in till lvl 30.
The general rule I follow is Act1 lvl 1-12, Act2 lvl 13-18, Act3 lvl 19-24, Act4 lvl 25-30 and pick up on Act5 from there.
Could it possibly be due to mercs ??
If your merc level is well below your level (and the monster level) and the merc does the killing, then your xp would be dramatically reduced. If, however, your char is doing the killing then the xp would be 'normal' for the character and you wouldn't see the drop in xp.
If this is in public 8 player games the difference could be even more dramtaic if other players mercs are doing the killing. Your xp would differ considerably depending on who/what/which does the killing and their respective lvls.
The only way to truly measure xp is in solo games where you are not affected by outside influences and can get a more accurate picture of how xp changes.
The only minion pack that gives low xp, in my experience, is the first lot - after that each monster gives about the same xp. Each pack gives different total xp due to the quantity of monsters in it, like Choogy says, and the balrogs are definitely more populous.
It would be very odd for xp to differ based on chacter - especially as it is not a published feature. How can lvling a necro SLOWER than a barb be considered 'balanced' ?? !! - surely it should be the other way round!
PS. tp to town, wait 20 secs and tp back is a good way of getting rid of Baal clone - works 90% time for me.