Plain strings are louder than wound strings, large strings are louder than small strings. B is the largest plain string if you have a wound G. Back in the early days of string winding I would wager the difference is more pronounced than it is now. Also if you compare it to the A string, compared to the radius of the fretboard it isn't all that much lower.
I think the reason they do it nowadays isn't to fix string to string balance, a flat stagger is close enough to perfect for the average person, but to make it look like they put thought into it, which makes it feel like they have a nicer product.