This answer to Mass ratio of solar-electric versus radioisotope thermo-electric power for propulsion; beyond how many AU do RTGs win? estimates a crossover at about 4.3 AU, so a trip to the outer planets will likely need a self-contained power source like an RTG or similar.
Suppose a future (as yet unspecified) mission wanted to slow down somewhat before a flyby of an outer planet. I'll ask a separate question about propulsion, but this means that it would need much more of its mass for propulsion for a given trip time.
Wikipedia lists the mass of the MMRTG at about 45 kg.
Could a similar design be scaled down in such a way that the mass-specific power output was similar? If a 45 kg RTG can produce 125 Watts at the beginning, could a similarly designed 4.5 kg RTG generate close to 12.5 Watts? Could a 1 kG RTG produce 2.7 Watts?
Answers to Requirements to orbit Pluto describe fairly large spacecraft, I'm asking here about going in the other direction.