It also is a question of strength.
The round shape distributes weight/stress evenly throughout the pouch.
A squared shape's corners would tend to focus that weight / stress.
The seams / stitching on the corners have to take that weight / stress.
Which makes those seams /stitching more likely to break.
When then break then your flask falls out of the hole in your sporran.
And you lose your whisky.
Therefore sporrans are round because the Scots love their whisky.
So now you know! 
Cheer
Jamie
(In all honesty I do believe it makes a stronger bag)
-See it there, a white plume
Over the battle - A diamond in the ash
Of the ultimate combustion-My panache
Edmond Rostand
Bookmarks