Please read in my previous message
"Some dolmens in Brittany not very far from the Carnac area"
"Some dolmens in the Carnac area".
It is not impossible that stormy weather is one of the more important factors explaining the lesser height of dolmens in, e.g., the Carnac area. Even nowadays, houses are frequently less high in regions with frequent stormy weather. This is even typical for certain regions close to the sea.