When you say "I know one is slightly over weight. I can see a little bit of skin between the scales, they are both perfectly rounded. More circle than a triangle..." it makes me wonder why you're asking this question in the first place.
As far as the size of the prey, there is no magical formula: the snake doesn't have to have the largest prey item it can manage to swallow in order to do well.
From your descriptions, I am confused about the age of your Ball Pythons... and I am also getting the sneaking suspicion that they are housed together. Regardless of what they are eating, their age, or their gender, it is best for your snakes if they are housed separately.
Every snake is an individual so pictures are really the only thing that will help anyone else determine whether your snakes are under weight, over weight, or ideal. In some individual animals, it is perfectly normal to see skin between scales at the widest point of the body... in other individuals, that IS a dead-giveaway that the animal is overweight, but no one here can tell you for certain which applies to your animal without seeing a picture, because it is the snake's overall proportions that determine that.
By feeding 1 or 2 prey items every week, your snakes could very well be an ideal weight. People who "power feed" usually are the ones who will advocate feeding the largest prey items possible, as often as possible - and that is usually because they don't just want to get the animal to breeding size, but they want to do it in record time to try to make a quick buck out of its first breeding. Unfortunately, those guys are also noisy enough about their successes that other people think that is the only way to keep these snakes successfully.
Breeders who care about the health of their animals strive for an ideal weight, just as pet owners should - that means they don't want to either over or under feed.
Other than this, your estimates of your snakes' sizes and proportions are a bit too loose for me to tell you anything more specific. Your feeding regimen sounds pretty good, but nobody can tell you anything one way or the other without pictures.
Good luck!
~Rebecca
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0.1 Dumeril's Boa '04 (Courtney)
1.2 Ball Pythons
[1.0 '05 Orange Hypo (Specter)]
[0.1 '05 Het Orange Hypo (Sylvia)]
[0.1 '03 Normal (Sue)]
0.2 American Pit Bull Terriers (40lb darling lap dogs:Brandy&Mara)