Size and shape of hay bales are dictated by the type and configuration of the baler employed to bale the hay.
In time past, all bales were small and oblong. This made them easier to pick up and carry around, to complete activities such as Stacking Bales in Stacks and Barns, and Unstacking Them Again To Feed to Stock.
Later, when automatic feeder-outer machines were invented, farm machinery manufacturers decided that bales could be made bigger; further, because hydraulics and front-end loaders had also been invented, bigger bales could be picked up and carried by large powerful tractors, instead of puny humans.
The first big bales were simply larger versions of the original small oblong.
Then came round bales, which are apparently more efficient. Certainly it is cheaper and easier to build a large round baler than a large square baler.
Distribution of various balers types - and hence bale types - is mostly dictated by the effectiveness or lack thereof of the salesman for any particular type, in any particular locale.
The cows don't give a shit, it's all just hay to them.