According to latané's (1981) social impact theory, as group size increases from two to three members, the impact will increase if a group increases from twenty-nine to thirty members.

The correct answer here is "More than".

According to Bibb Latane and his Social Impact Theory that he developed in 1981 the impact will increase more if the group size increases from two to three member. This may sound strange but according to his theory the more members the group, that is targets of impact as he called them the less impact  individual targets have. So that means that in larger groups the impact of a new member is weaker than in smaller groups.