let's start with some basic assumptions :
- In space, thermic exchange are made by IR radiation. It is ruled by the Stefan-Boltzmann's law, sigma*€*T^4
- Due to solar radiation, an object is naturally heated.
- By insulation, you can slow thermic exchange, but you cannot stop them. You can concentrate heat on a thermal sink for some amount of time if you want.
- Everything generates heat : human bodies, cooking, recycling Co2, etc...
In conclusion
Even without internal thermic generation, an object in space has a thermic signature.
Without an internal heat sink, you will heat up to a temperature which allow you to radiate enough heat to compensate for the generated heat. And the relation is not a ^2, but a ^4.
Heat generated has a component scaled to the crew (body heat, survival gear, cooking, and so on)
Efficiency and reactor tech have an impact as the more efficient a powerplant or engine is, the less heat it generates per Kw/h.
On a technobable point of view, the reduced thermal tech is some sort of thermal sink coupled with radiator which can store then radiate heat on a directional path, to avoid detection.
But if you consider a base thermal signature, IMHO it must be scaled to the crew and not the size of the ship, as you can cut nearly everything on a ship but life support.
A fun addition will be to have for a limited time the possibility to set a thermal reduction order for ships : the crew get into spacesuit, breath stored oxygen, and eat protein bar. It will reduce efficiency, but will lower the thermal signature, at least for some days...
My 2 cents.