The system is designed so that optimally, there are 4 members in a party. That is in-line with their statements that they want the norm to be 4, and the megaboss fights to be 8. In fact, adding any more members above 4 on the field really does kill your sp per hour. It's not a marginal decrease, it's a 15-25% decrease in SP per hour.

This is unfortunate, because 4 meleers at ants that can heal themselves is much better SP per hour than 4 meleers and a healer.

It's not a marginal sp reduction. 30 sp difference per mob is a big difference in sp per hour when the mobs are dropping a base of 175.