The rate of radiative cooling scales proportionally to (T^4-Tenv^4) which approximates to just T^4 in space (Tenv = 3K). The hotter they can run it, the smaller heatsinks they need; for every doubling of temperature, the heatsink area can be reduced by a factor of 16. Also, it might be possible to boost the output temperature, e.g. with a chemical heat pump for even smaller heat sinks.