Honey08
Waffled a lot!
Mammal that have evolved to live in cold areas have small ears (to retain heat) and those that have evolved in hot areas have large ears (to cool them). It's as simple as that. Your mare is almost certainly part Welsh, as welsh ponies and cobs have disproportionately small ears.
Probably the real reason.
I was told if you plug a cable in to a big eared hoss's ass, you can get freeview on your iPhone.
When I was shown around the Heathrow T5 lounge before it opened, they had two lifesize model horses at the top of the escalators with lamps on their heads. It caused much humour on the "showrounds" to see that the flex did indeed come out of their bottoms! They probably cost a fortune, but were hideous!