That's a great set of images to illustrate the compression effect. Isn't that awesome the way that large tree to the right just comes in there so big? Here's the "why".
Distance from the subject and magnification are directly related. Let's use easy numbers to illustrate. Suppose you are shooting the gnome (great test subject by the way) at the shortest zoom length of 55mm on your lens and standing 20 feet away from him. You take a shot. Now, you zoom out to 110mm on the zoom and look through the viewfinder. You have doubled the magnification (55 X 2) of everything in the view finder - the gnome the trees, etc. Double magnification means everything is now twice as big in the viewfinder. The way you get the gnome back to the original size while shooting at 110mm is to now double your distance from him. So you move back from 20 feet to 40 feet. You have doubled your distance (20 ft X 2 = 40 ft) to the gnome and now he is exactly the same size in the view finder that he was when you started. Doubling the distance cuts his size in half, or 20/40 = 1/2 (or .5).
But what about the background? Remember I told you to make sure you had plenty of distance behind the subject? Suppose that line of trees back there is 50 feet from you when you take the first shot? Remember when you zoomed in, you doubled the size of the trees too. But when you move back to resize the gnome, the distance from you to the trees is going from 50 feet to 70 feet (the original 50 foot distance, plus the 20 you just moved). So the magnification factor of the trees as you move back only changes by 50/70 or 5/7, which is .7, which is a long away around to say that because the trees are farther away, they don't shrink as much when you move back..
And the effect actually gets more pronounced the farther the background is behind the subject. Suppose the trees were 100 feet away when you started with the first shot. Then you zoom in and they get double in size. But when you move back, you're now 120 feet from the trees and the change in magnification of the trees as you move back is only 100/120, or .83.
And in your case, you zoomed out from 55 to 250, so you changed the magnification of the gnome by a factor 5 when you moved back but the magnification of the trees wouldn?t have changed nearly that much.
You can see if you keep putting more distance behind the subject for the first shot, then zoom in, that you eventually get to a point where the background won't appear to shrink at all when you move back. If the background were a mountain several miles away from you and you zoomed in after the first shot, looked at the mountain, and then moved back, the change in the size of the mountain in the viewfinder just before and just after you move back will essentially be zero.
It's the same reason that if you were looking at a setting sun in your rearview mirror while driving directly away from it, the apparent size of the sun woulnd't change at all, even if you drove several miles. It is so far away in comparison to the distance you are driving that the the relative change in distance is miniscule and thus no change in the apparent size as you move.
We can take advantage of this effect when we want to fill the view finder with something in the distant background. Zoom out to a longer vocal length to fill the viewfinder with the background, then backup to get our main subject back to the size we want in the view finder. The subject will shrink back down by quite a bit while the background shrinks down by a comparatively smaller amount.
The reason the bokeh is very similar is bokeh depends on both the focal length and the distance to the subject. Longer focal lengths give more background blur. So do closer subject distances. But in this case, the move backwards from the subject while zooming in for magnification for the second shot offsets the effect of going to the longer focal length.
The "zoom" really has nothing to do with this. It's just a matter of focal length, or magnification. So if you stood with 2 cameras (both either full frame or crop) in the same spot, one with a 135mm prime and one with a zoom set to 135mm, you should see the same level of background compression in the view finder.
The way we will use this going forward is understanding that the best results for backlighting are usually obtained by having a dark background. That tree line you have behind you would actually be a good set up. But we don't want the subject too close to that background. First, we want the background blurred so having more distance behind them will help that. Second, we'll usually be shooting later in the day when the sun is lower in the sky and if they are too close to the tree line, the sun will be blocked by the tops of the trees. So by taking advantage of the lens compression, we get the viewfinder filled with the darker background, but still have our subjects far enough away from the trees to catch the sun peaking over their tops, as well as a nicely blurred background.