Well, almost. Its not the bushing that allows the slide to be made so thin, though. Its because it has internal locking lugs. So does that Hi-Power of yours. Both the Hi-Power and 1911 slides are thin.
Sorry, and all due respect to you, but this is wrong.
Yes, both the Hi-Power and 1911 slides are thin, but compare the two side by side and you'll notice that the Hi-Power's slide is no thinner than that of the 1911, despite the fact that the barrel inside the Hi-Power's slide is of significantly smaller outer diameter than the barrel of the 1911. The reason for this is the barrel bushing on the 1911 and the lack of it on the Hi-Power.
The locking lugs do indeed allow the upper surface of the slides of both guns to have a rounded off contour, however they do nothing whatever to affect the width of the slide. It is the barrel bushing or lack thereof, and how this allows the barrel to be removed from the slide that affects that.
As I said earlier, when the barrel bushing is removed, the 1911's barrel is removed by pulling it longitudinally out from the front end of the slide. If there were no removable bushing, and the 1911 had a solid front end like the Hi-Power and most pistols since have, then the barrel would have to be removed the same way the Hi-Power's is -- by lifting it out through the opening on the underside of the slide.
Now field strip a 1911 and try to remove the barrel through the underside of the slide. You can't. It won't come out that way. The slide rails are too close together to allow the barrel to pass between them. In order to get the barrel out between the slide rails on a 1911, the rails would have to be moved farther apart, probably about an eighth of an inch, which would necessitate widening the entire slide by the same amount. The result would be a thicker slide. This is why the .45 caliber M1911 has a slide that is narrower than most 9mms, even though the 9mms, with smaller-diameter barrels seem like they ought to be flatter through the slide. They're not because their slide rails have to be spaced far enough apart to allow the barrel to come out between them. So even though you have a slimmer barrel, the slide is as wide or wider than that of a 1911.