Another different Sanyo datasheet reflects echowars' aforsaid cites for the STK-0080 and STK-0105 specification for 8 ohm loads.
My interest continues with the physical and electrical design of the STK-0105 as distinguished from the STK-0080. A further closer examination of the Sanyo datasheets reveals why the STK-0105 is quite a bit physically larger than the STK-0080:
In detail, the specific electrical and mechanical differences between STK-0080 and the STK-0105 are as follows:
The physical outline for the STK-0080 is 64 millimeters wide and 39.5 millimeters tall with a fastener distance of 55.6 millimeters center to center:
The physical outline for the STK-0105 is 78 millimeters wide and 44 millimeters tall with a fastener distance of 70 millimeters center to center:
The internal electrical schematic for the STK-0080 is a pure complementary circuit of 5 transistors in a darlington configuration with 2 emitter follower outputs:
The internal electrical schematic for the STK-0105 is a quasi complementary circuit of 8 transistors with 4 emitter follower outputs. Thus, in short, the answer why the STK-0105 is substantially larger than the STK-0080 is because the STK-0105 is physically housing twice as many high current emitter follower power output devices than all of it's other STK siblings:
Therefore, the STK-0105 is clearly capable of better power output performance than it's STK-0080 "little brother" which has only half the emitter follower final output devices.
In the future, I am definitely going to remove the lesser capable STK-0080 devices and use the original specified STK-0105 devices in any Fisher RS-2010 repair and/or restoration project. The difference in cost is only just a few bux, if you know where to look for them. The power output of the STK-0105 should be greater and long term reliabillity should also be substantially better.