More xp out of party.

[Asanarama]

Companions and hirelings outside the party each get (about?*) 25% less xp than each character in the party. (I'll just use "companions" after this, but it always refers to both.) Also, the smaller your party, the more xp you get. If you are solo when you gain xp, your main character gets 50% more than it would with a full party. If you remove everyone from your party before gaining xp (a total no-brainer for some quests), companions each get 12.5% more xp than they would if five of them had been in the party, and your main gets the 50% more. This is consistent with the information on the wiki, but it doesn't mention these kinds of results.

In a party with four companions, each party member gets a 10% bonus compared to a full party, so compared to an empty party, companions get only 2.5% more xp, and your main gets 40% more. In a party with three companions, each member gets a 20% bonus, so compared to an empty party, companions get 7.5% *less* xp, and your main gets 30% more.

* In my test, which consisted of several quest steps and turn-ins, and location discoveries, my main gained 1455 xp and the others gained 1082, a 25.64% difference, probably due to rounding. Also, the log seems to display total xp given to the party, not including the bonus, but that's something I just noticed a few times, didn't check carefully.

P.S. Things get weird if you care about exactly four companions and want to lean towards giving them more xp and your main less, to reduce the xp gap. Keeping two in your party with your main for turn-ins results in an average increase, not counting your main, of 13.75% xp instead of the 12.5%. You could rotate members to keep their xp consistent. Very miniscule increase, but mathematically interesting. Same exact numbers for companions if you keep just one in your party with your main instead of two (but your main gets more).