One of the big stumbling blocks that many individuals miss when moving up from small lathes, is a much wider availability of accessories/features like chucks, QCTP, tooling, inserts, etc. Usually when you get into the 12X and beyond, you get into standardized chuck mounts and tooling. Both from a new and used market, this opens up many more options and sometimes is more cost effective. Also it is not necessarily swing diameter, but spindle diameter, gearing/gearbox, rigidity, accuracy, etc. that are significantly improved by graduating out of the mini/small lathe (10" and under) class. I have a 13x40, truth be told I would have preferred something like 1330 or 1430.
I do a lot of small work, under 1", but when turning heavier stock say 4-5" you want some mass behind your lathe. I use a 5C collet system probably 50% of the time, so usually mandates a D1 type chuck mounting system, or a larger MT through the spindle setup. On a smaller lathe you could probably adapt some ER collet system, but you are often very limited as to getting back plates, let alone a quality chuck and accuracy. So not always size but all the other features that make the lathe much more versatile/usable. You also have things like separate feed and lead screw, the list goes on. Both the PM 1127 and the 1128 lathe are basically the break point minimum as to commonly lathe features and more universal tooling, the latter having much fewer change gears. When I was purchasing my lathe, I had no prior lathe experience, so it was hard to conceptualize that which I hadn't experienced. In the end others provided guidance and their experience, I ended up with a 1340GT about 4 years ago. My recommendation to others, is to factor in all these other benefits when upgrading, I am sure glad I did (although it doubled "my budget"). I would have already upgraded in a few short years had I purchased a smaller lathe. If I ever upgrade, it would probably be to a heavier 13 or 14" lathe with a 2" spindle bore, but that is more dreaming. So for me, it was a long term investment but I also understand that one needs to be mindful of the budget.