i think it's this part that control the exp gained from your situation..
trunk/conf/battle/exp.conf
// Method of calculating earned experience when defeating a monster:
// 0 = uses damage given / total damage as damage ratio
// 1 = uses damage given / max_hp as damage ratio
// NOTE: Using type 1 disables the bonus where the first attacker gets
// his share of the exp doubled when multiple people attack the mob.
exp_calc_type: 0
// Experience increase per attacker. That is, every additional attacker to the
// monster makes it give this much more experience
// (eg: 5 people attack with 25 here, +(25*4)% -> +100% exp)
exp_bonus_attacker: 25
// Max number of attackers at which exp bonus is capped
// (eg: if set at 5, the max bonus is 4*bonus-per-char regardless of attackers)
exp_bonus_max_attacker: 12