Thermal mass effect is often defined as thermal capacitance or time lag. It is the high heat retention factor of a solid wall to limit heat transfer through itself. A solid log wall in summer will delay the heat trying to get through to the cooler inside of the cabin.
In 1980 - 1982 the National Bureau of Standards in the US did a study for the Department of Housing and Urban Development (HUD) and the Department of Energy (DOE) in an effort to determine the effects of mass on thermal retention.
The NBS compared a log building to an insulated conventionally timber-framed building. The log building was constructed with 140mm solid logs rated at a nominal R-10 and the framed building was insulated with 70mm of fiberglass and rated at a nominal R-12.
The NBS found that over the winter heating period the two buildings used basically the same energy for heat. But over the summer cooling period the log building used 24% less energy and over the spring heating period the log building actually used 46% less energy.
One problem with using R-values alone to compare the thermal performance of materials is that R-values are only a good representation under conditions of constant interior and exterior temperatures. This is just not the case in the real world. The R-value has a inverse correlation to the temperature differential.
Remember there are three types of heat transfer: convection, conduction and radiation and fiberglass insulation basically addresses only convection. A solid log wall also resists the conduction of heat.