yes, while space is cold, the vacuum means that nearly all the heat has to be shed by radiation (meaning e.g. infrared radiation, not nookyular radiation) rather than conduction & convection. so the more you scale it up, the bigger the heat sinks needed. think giant metal fins to radiate away the heat. eventually it becomes prohibitive.

interestingly, the 40KW NASA pilot project this proposal appears to be derived from is a Stirling engine, not a radioisotope thermal generator like on New Horizons, Cassini, etc.

a Stirling design converts much more of the heat to electricity than an RTG, but it has moving parts.