I nursed 2 babies for 3 years apiece and never once felt the letdown sensation. Some women have it, some don't.
Also very much that just because a baby WILL eat 5-6 oz at a go doesn't mean he actually NEEDS all that milk. I once fed my 3 week old baby almost 7 oz of formula after nursing her, because she seemed so hungry. I mean, she nursed, then chugged down a 2 oz bottle and then screamed like she was starving. So we gave her another 2 oz. And another. Finally she seemed content- and then she started to fuss again, and after a few minutes she puked up at least 5-6 oz of what we'd fed her. Yuck! Babies love to suck and they need to suck. At the breast, a baby can suck for comfort without eating much if anything. With the bottle, the baby cannot suck without getting milk in her mouth, and then she has to swallow or she will choke. So it's very easy for a bottle-fed baby to take in way more milk than she actually needs, and if she does that often enough, her tummy is going to stretch out, and she's going to come to expect that stuffed-to-the-gills feeling every time she eats, leading to her crying even after a perfectly adequate feeding.