From personal experience this seems to be true, the higher off the ground compared with the distance from the fire means more loss of heat.
Obviously if you have a good heat reflector then you should have no problems, but a small fire controlled correctly can issue enough heat to keep people warm as long as thier proximity to the the heat is not that far.
This also depends on enviroment and legal restrictions for your area (Can you start fires legally and can you control it so that it causes minimum impact on the enviroment), watch out for byelaws, they can be very tricky, the ones on Dartmoor have a considerable effect on me.
Legally BCUK cannot condone fires which have not been legally agreed and as such cannot condone any fires that are set by members unless they have been agreed legally by those concerned.
However a low camp will keep heat in very effectively if constructed correctly, star fires are very efficient (I sound like an echo) they give all round heat and give you the option of increasing the heat directionally and adjusting the fire type as well.
Be legal and safe (on Dartmoor for me without permission it could cost as much as £1500, if caught without authority and getting authority can take a long time), speak to the local authorities, get local people as references for you and you may get chances of camping in the wild with impunity if you abide by the law.