My suggestion would be to never use a swing without a spring. Contrary to marketing babble, the springs serve a real purpose on impact forces. The springs are there to act as dampeners and reduce risk of falling from breaking ceiling joists or stands. This is much the same reason that if you look at the tub in your washing machine, it is held in by a bunch of springs and not fixed points. They are not trying to make the washing machine have more fun, they are trying to make it not fall apart from the impact forces caused by rigid mounts.
So never use a swing without a spring. If needed get a stronger spring.
Also for plus size, the biggest issues are the width. When you sit in them the straps will pinch in on your hips. The wider the swing the better. Examples would be the pleasure swings by whip smart are only 14 inches wide, the trinity, tlc and fantasy swings are 19 inches wide, the screamer is 19 inches wide and there are a few dual hook swings such as the screamer dual hook and the deluxe pleasure swings that can be set to any width you want.
I you want comfort, you want a wide swing.
Lastly is pressure from the strap. Granted they all have wide padding, but the padding wraps around the webbing when you sit in it, so the effective strap width is essentially the same as the underlying webbing. Wider webbing is more comfortable.
Now the Pleasure swing by whip smart is only 1 3/4 inch webbing. The trinity, tlc and fetish swings are 2 inches. The screamer is 4 inches and there are several door swings that have seats up to 8 inches wide.
So basically - always use a spring even if you must upgrade, wider swings are better, wider straps are better.
Hope that helps