We used to have a "short haul" and a "long haul" diaper bag. Eventually it basically ended up at: a couple of cloths, extra breast pads, enough diapers for how long we thought we'd be gone plus an extra, pack of wipes (I could never get into having just a few wipes in a plastic container, poop always seems to take a zillion wipes and I never know when one will strike, but some babies are really predictable poopers so I suppose some parents chance it), bag to put used diapers in (we use cloth), pacifier, and maybe a couple of toys. We rarely used a stroller in her first year but were never without the Mei Tai or Boba. I'd bring a blanket if we were going somewhere I thought I'd put her down on the floor, up until she got old enough to crawl off the blanket at which point I stopped bothering. We fit everything in a backpack easy with enough room for me to take my purse, book, or whatever. I think huge diaper bags encourage parents to pack their whole life in the bag. And parents who use bottles will legitimately need to bring more stuff.
Now she is 20 months old and if we will be gone less than 3 hours I just take 1-2 diapers, pack of wipes, bag for used diapers, a water bottle for her, and a couple of toys if we are going some place that doesn't have toys and where we will be stuck for a while. We are well past the extra cloths and extra breast pads stage, but those things saved me so many times early on. Also we used to travel with a changing pad early on, but now I figure the odds of her peeing/pooping when the diaper is off are very low since she hasn't done so since she was a newborn. Unless we are going some place involving a lot of walking, like the zoo or the mall, I don't bring the Boba or stroller because she walks nicely for short distances now. At the grocery store she sits in the cart.
If we're going for a walk at HER pace I don't pack anything at all because we never make it more than about a block away. If we were in the stroller and I was planning to walk at MY pace I'd probably bring the aforementioned items just in case.
You will figure it out, I promise! We had our fiascos early on, but we got the hang of it.