One of the primary arguments of a keto/paleo diet is that the different diets affect how many calories your body thinks it needs, how it uses them, and how it influences behavior as a result. (Disclaimer: using imaginary numbers below to illustrate the theory.)
Let's say your body typically wants 1000 calories at breakfast, and you eat 1000 calories with a heavy load of carbs. Carbohydrates with a high glycemic index cause your insulin to spike. That signals fat cells that they should start storing energy, and they do so, tucking away 300 of the 1000 calories you ate.
This means your body only gets to spend 700 of the 1000 calories you ate, and as such, it says "hey, I'm still hungry". You eat 300 calories worth of food. But your fat cells are still sucking up (X%) of what you eat into storage, due to the insulin reaction. So, your body gets 200 of those 300 it wants, and it stays a little bit hungry. (Or, more likely, your body wants 300, but you eat 500 to make it shut up.)
Carbs have the unfortunate habit of converting useful calories into fat storage prior to processing them for the purposes of nutrition. This means carb-heavy diets tend to cause unconscious overeating, and also constant feelings of hunger/cravings/etc.
One of the advantages of a paleo/keto diet is that they often avoid the type of carbohydrates that cause this problem, namely ones with a high glycemic index. This means when you eat 1000 calories, your fat cells don't skim any off the top before your metabolism gets to them, and you get the full 1000; this means you don't end up hungry after a meal, and don't suffer from the urge to snack/eat more. Because your body got all the energy it wanted, it doesn't start saying it's hungry again until it actually does need the nutrition, and you're more likely to eat closer to the correct amount for your body's needs.
The difference between the examples you give, 4500 of Paleo and 1200 of junk food, is primarily how they'd make your body react. 1200 of junk food would certainly be a caloric deficit, but you'd be ravenously hungry at that level. (I've done 1000 calories a day for 6 months straight - it's pretty awful for the first few months.) But with keto, you could eat 4500 calories in a day, but you won't want to. When I'm done eating a keto-style meal, I am completely uninterested in food until the next. Those meals, for me, are typically 1-2 small/medium brats with no bun. But if I go out and cheat, and grab a burger and fries, I've got to fight the urge to follow it up with some ice cream, even though the fries and burger combined are drastically more calories than the brats I was satisfied with the meal before.
It's really interesting stuff. The "calories-in, calories-out" model is completely right from a completely energy-based perspective, but it doesn't account for the side effects produced by the energy source, and how people react to them.