The difference between published MERs and implied MERs is due mainly to transaction costs, which are a factor of a fund's turnover rate. The increased cost puts the heavy trader at more risk of underperforming. High turnover is sometimes unavoidable. Specialty funds that focus on specific countries, regions, or sectors will almost always have high turnover. That's a natural by-product of the narrow mandates of those funds.
Larger companies have a cost edge over their smaller counterparts.
The large amounts of money managed by bigger firms allows them to negotiate lower trading costs, thereby resulting in a less severe impact when trading securities. Even if a fund is small, a firm may have a substantial institutional business in order to negotiate low trading costs. These issues are not restricted to mutual funds. They apply to all types of pooled investment funds and wraps that use investment funds. As mentioned above, this information not easy to find.