As with all other unit types, pioneers default to larger and more expensive groups as you research the appropriate technology. At present, those larger groups are added expense for no added advantage. I would suggest that some advantage be given to larger groups of pioneers. Possibilites:

• Cities founded by larger groups of pioneers start with more population, or a higher level, or a free building, or a larger area of influence.
• If roadbuilding is put in, larger groups of pioneers could build roads faster.
• Outposts founded by larger groups of pioneers could have a larger sight range (but not larger area of influence) or a defensive advantage if attacked.

If larger groups of pioneers are to remain only more expensive, please change the default pioneer group to the smallest possible.