I agree w/Paperprincess generally. I think much has to do with the bag's 'purpose' as well as size. Smaller bag; smaller opening?? But then again, if one is going to toss the bag over their shoulder - even if a smaller size; not 'shoulder' bag - it might need a wider opening?? IDK.
I'm not a bag/purse person myself. Essentially, if it doesn't fit in my shorts/jeans pocket - I don't carry it!