I noticed quite a bit of variation in my first few games. I would agree that barbs taking settlers probably accounts for some of it.
Biggest one I've had so far I was Byzantium, on my continent were Carthage (right next to me), Egypt, Denmark, Sweden and Austria. Egypt stayed in one city and wonder spammed, then VERY sensibly joint declared war on me with Austria with no army. A nice extra capital with 3 wonders for me then

Austria put a city right on my borders, I burned it - her little bit of land seemed to have a lot of very tall cities on it too but she was quite boxed in and haphazardly flinging cities all over the continent whereever she could find space. Over time, Carthage spammed cities all around me before I took them, Denmark stayed almost on one (and settled a GM beside Copenhagen) before LUDICROUSLY backstabbing me while my entire army was already mobilized on his border fighting Carthage - he was left with just one rubbish polar city.
Sweden were interesting - they went to 3 cities very quickly, but there was a huge expanse of land between me and them that they just didnt fill in at all. I've now got the entire left hand side of the continent and the tip of the right side where Egypt were, Sweden are middle right, Austria bottom right, but theres a huge expanse right in the centre that'd be prime for Sweden to move into - or Egypt, had they survived I guess. On the other continent everyone seems to have expanded fairly normally, they've got far more land and a lot of big civs.
So it might be partly flavour, and partly barbs taking settlers. Egypt seemed to be very clearly choosing to stay in and turtle up, but utterly neglected to build defenses or an army. Austria liked being wide, as did Carthage (despite lack of space), but Sweden built a very compact core and wouldnt expand further.