Really easy thing to science on actually. Err, !!SCIENCE!! -- apologizes to DF.
So, standard 7.1 Earth start. 500m pop, 420 factories, and so on. 12K+ Thermal, 35k+ EM signature.
If we quadruple the pop to 2 billion, leave everything else alone(SM is your friend) -- 19k thermal, 110k EM. So population affects both.
Back to 500m pop, and we'll nuke all the construction factories and mines. 8k thermal, 31k EM
One could go further in this regard, but it's safe to say both industrial development and population affect both thermal and EM signature. It also seems that EM comes mostly from population. And so on ...