Level should not matter.

If two 20's kill a mob on their own, you might get 1,000 exp each.
If the level 80 is in the group, you might get 200 exp on each of the 20's.
But the level 80 can kill 100 mobs in 5 minutes.
The two 20's would down maybe 10-15 mobs.
And with instance resets, the 80 won't run out of mobs.

There is a maximum distance in which the lowbies can be from the kill, and still get experience.
In 3.3 (most recent content patch), that distance was drastically shortened.
So you'll have to move the lowbies up a lot closer to the action, and move them closer more often then before.

Not sure why mobs would disappear so quickly.
Are you comparing the 67 to the 80, now?
Or the 67 previously, to the 80 now?
I'd suspect the 67 now, will be about the same as the 80 now.

Basically kill freely, but observe the lowbies.
Once experience stops, move the lowbies up to almost where the high level is.
And kill some more, until you need to move the lowbies up again.
After a run or two, you'll know how far you can kill before you need to adjust positioning.
And then the runs are a lot faster.

You should get more experience per kill, with the 67 killing then the 80 killing.
Basically your booster is getting an even share of experience.
I'm not sure the actual formula.
But (107 - 67) divided by 107 is a larger number then (120 - 80) divided by 120.
However the 80 will kill a lot faster, so you're better off with the 80 regardless.