Maybe I've misunderstood, but calculating XP doesn't change based on party size. Whether there are 2 characters or 20, you figure the average party level to determine total XP for an encounter, then divide that total by the number of party members. Parties with 10 members will gain XP much more slowly than parties of 4, without any need to fudge with average character levels.
That said, if you have a large party with significantly different levels, I would heartily recommend Winterthorn's suggestion to break it into two or more "chunks". So long as everyone is at risk in the encounter, it seems fair to award XP based on different average levels.
Where party size matters is in encounter level (EL). A party of 8 characters can handle roughly twice as many enemies as a standard party of 4. This may result in EL's a couple levels higher than the average party level just to make them break a sweat, much less challenge them. But be careful not to simply throw a higher-CR creature at a large party - the creature may have weaknesses that lower-level characters cannot take advantage of, or strengths the party cannot counter, resulting in a TPK or near-TPK.
Example: a gargoyle is a CR 4 creature and has DR 15/+1. If you throw one at a party of 8 2nd-level characters with no magic weapons, how difficult is the encounter really? A better choice is to use bugbears or ogres and simply increase the number of opponents.