I had hoped to be able to write up the origins of the phrase "to broaden one's horizons" but, disappointingly, my usual sources failed to even mention the phrase, let alone provide me with an explanation of where it came from. (OED, Brewer's, Bartlett's) The only context in which I'm used to hearing about multiple horizons is astronomy, but I can't think why an astronomer would require bigger horizons than the sky provides. What would "narrower horizons" be? The view out of a window, as opposed to being outdoors with no trees or hills?
Bartlett's at least had the phrase in a couple of permutations, the oldest of which was written by Baudelaire in the mid-nineteenth century:
" To be just, that is to say, to justify its existence, criticism should be partial, passionate and political, that is to say, written from an exclusive point of view, but a point of view that opens up the widest horizons."
haggisthesecond and I have concluded that, all other things being equal, shorter people have broader horizons than taller ones, since the higher up you are, the closer far things appear to be.