I think it's simple physics - the lighter, half-ton towable trailers have too much surface area relative to their weight. Less weight and more surface area equals more sway.
I had three different trailers I towed with my half-ton Suburban. All were 26-30 feet, weighing from 4550 to 7000 lbs. All three gave me the push-pull with passing semis, and all were susceptible to moving me around with varying wind gusts. On one trip on a windy day, I had the sway brake cranked down so hard that I bent the half-inch steel plate that the sway ball was mounted to.
I now have an 8600-lb trailer I tow with my 3/4 ton Suburban, and it doesn't sway. Period. No push-pull with the semis, and I don't even notice crosswinds. I've pulled the trailer down South to the Gulf, over the Mackinaw Bridge, and out west in the Rockies, including Utah, Colorado and Wyoming, and up and down the Front Range which is known for high winds. I don't even use any type of sway control. Have towed this trailer about 10,000 miles, and never had any type of stressful event above a 3, on my scale of 1-10. Had a few 8s with my lighter trailers.
If I ever had to go back to a lighter trailer, I'd go with a hitch that eliminates sway like the Hensley Arrow.