I remember somewhere Robin saying that population density on Harn was very low so buildings were on the larger end of size limited by architecture...so peasant hovels were as big as you could build em before the roof collapsed
. At least outside cities that is; where defensive lines/walls/population did not limit space.
It reminds me of some of the villages in (wealthy) southern Britain when I cycle round them...a few tiny cottages (for the cottars) but in the main large barn like houses spaciously set around a village green; large sateliite farms..and a very large stone walled farm/manor complex. I am sure these villages are a shadow of what they would have looked like in medieval times...half the houses are barn conversions and I am sure many cottages did not survive, especially near the outlying farms etc.
As for other reasons..I think the fact Harn is probably more clan based like pre and early medieval societies means buildings are more likely to be large clan like halls than the smaller 'family' houses of later times. (Although if more people live there this should not increase living space......
So how about this.. timelag architecture...the tradition and knowhow of house building is for big clan houses...which until recently were for larger clan units.....but now are being built for smaller satellite units. There is plenty of raw material (explicitly stated in Canon) and space. Which means for now the lucky Harnians are getting de-luxe apartments for their buck! I would suggest the possible inefficiency of this (In man hours/heating costs) is offset by having space for wintering larger amounts of livestock and creating dry workspaces in the damp Harnic weather; as well as being supported by the innate conservatism of the community. (we have always build em this size!!!).
This would make Harnic communities (as the population increases) very land hungry and probably growth obsessed...aaaarghhh the end of the beginning is nigh!!!!!! Do not worry though...Clan economists and lords who can write and add up will soon be realising that if they got these mini clans to live in smaller houses..there would be more land to farm and more man hours to do it.....
So for short..I do not know the real reasons in full.