I never much got the "not enough space" argument. I live in a 580sqft condo, grabbed a turkey burner that I use on my patio, and BIAB all-grain. At the moment I've got 10 gallons in 3 kegs serving, and 25 gallons fermenting in my living room, out of the way. My brew day is 4-4.5 hours, from strike to end of cleanup.
If I didn't want to buy the turkey burner, my only investment would be the pot (MLT and boil kettle in one) and I can get that to a boil nicely as it sits over two of my apartment-size stove burners. I could potentially still be doing what I'm doing.
That said, my first batch was a 1 gallon and I still have that jug. Definitely cool for experimental batches. I guess if you're absolutely intent on not buying anything other than ingredients, 1 gallon batches make sense, but if you have a 7 or 8 gallon pot already, that's all you need to do bigger batches. I like the portability of 1 gallon, though. I can't exactly pick up my buckets and just drop them in another room if we're having company. That's a bit of an endeavor, and bottling 12 bottles instead of 52 certainly has its appeal.