I go to 100lvl dungeon today, and the lowest level member says we are all work for the highest level member, because our level is lower. (We got level 132, 123, 119, 109,104 in our party.)
But as i knew, there is a Equal Distribution mode in party setting.
So is the highest level member get the more exp in the dungeon or what?
Oh of course we are setting as Equal Distribution mode. 
Equal distribution is Equal distribution.
Lower levels tend to work harder as they do not have the same damage as a higher level character.
Regardless, the EXP is distributed equally as long as everyone is within level range of the dungeon.
I ran a level 50 dungeon as 81, didn’t get any exp but everyone else did just fine.
If you’re that concerned, you can always ask the higher level people if its ok to switch to By Level distribution. this means lower level people will get more.