The more cash you put in, the more the property costs you. With positive CF, the tenant is paying the interest, not you. The total cost of the property isn't equal to the total cost to the REI,...or at least it shouldn't be.
Buying equity isn't making money. All you're doing is moving your cash to the property. It's the same money, you just froze it. Profit is made after you recover your costs...which is ONLY your cash you put in. You recover that cash from your CF. The more cash you put in, the more cash you have to recover, and the longer it takes.
On the other hand, 100% owner financing with no interest is great. That means you got the property for nothing.