According to wowwiki it says that xp formula when grouping with a lvl difference is:
XP1 = MXP * CL1 / (CL1 + CL2), the experience awarded to character 1

So if mob gives 100 xp and character 1 is lvl 1 and character 2 is lvl 13, then it should give 7.14 xp.
If it gives 100 xp and character 1 is lvl 1 and character 2 is lvl 1, then ti should give 50 xp.

If the same formula is used for bigger sized groups, then it is 5.88 xp with 1 lvl 13 and 4 lvl 1.
Or with everyone lvl 1 it is 20 xp.

So according to this formula there is a huge drop off in xp.