As I understand it (from a conversation with an engineer many years ago).
Buffers are quite a heavy piece of steel, and have to take some considerable thumps of course. However they also need to hang off their internal springs/oleos, so you want to keep them relatively light to avoid overstressing these downwards, and therefore keep the buffer face relatively small.
Against this is that if you make a circular buffer face too small you have the potential for "buffer lock", particularly on sharp reverse curves (eg over crossovers), where the buffers of the two adjacent vehicles manage to move out and then behind one another, and as things straighten up the lighter of the vehicles can even be twisted into derailment. Therefore a horizontally-oval buffer shape tries to meet both requirements. Getting vertical buffer lock is less likely, although not unknown.
Tenders of steam locomotives were particularly prone to this because of the considerable difference in their weight between being loaded and empty, and therefore their relative height due to their springing (I know anyone who has stepped back from the footplate to the tender when at speed might deny that the tender has any springs ...)