Taper is one of the most overlooked issues with tree pruning, and perhaps causes the most problems that allow trees to get destroyed by wind or snow. The problem is that a trunk or heavy branch can't be strong if there's not enough taper, or if there's too much weight only at the ends. It can't always support the weight it's holding including its own, especially if there's fruit or heavy flowers. And it really just doesn't look right and shows a history of neglect or improper pruning. If you want to keep a tree a desired size, at least in the near term, it's imperitive that you encourage tapering so that once it's the desired size you can't go back and start over. Tapering gives you more options.
The tricky part is determining how much taper there should be. A trunk and main branches will ever increase in diameter over the years, and you can't stop it. You can slow it down or speed it up with various techniques. Eventually, the trunk diameter will be a good proportion to the overall height, but if the taper is lacking, it won't look right no matter what the base diameter.
There are some different patterns that dictate the amount of taper to a degree. Willows and maples and some birches and various other trees are known to be more slender, so the amount of taper isn't as obvious. But take a look at an old oak tree, and you'll usually see great taper throughout the tree, unless someone has pruned it incorrectly. That's one of the reasons the trees look (and are) so strong. Like a muscleman with thick upper arms, tapering to fairly thin wrists, instead of looking like Popeye.
In bonsai, taper is usually much more encouraged. Some of these old trees are merely 2 or 3 times as tall as the diameter of the trunk, and the trees look very old and permanent and strong. I have a little olive tree with about a 3" diameter base that's only about 10" or 12" tall, but I got it that way from a previous bosai grower and frankly the taper is too abrupt and needs refinement. I strive for a minimum 20 to 1 ratio in all my trees in my collection. In landscape trees, however, you'll never achieve a ratio of probably 10 to 1 or better in your lifetime, at least not in a good way.
As a very rough estimate, without studying many trees with a tape measure, I would probably see good trunk taper as anything more tapered than about 30 to 1, height to base trunk diameter, and 20 to 1 is really good. Once again, I wouldn't expect the latter thickness to be common on a Japanese Maple or a Willow, but on more stocky trees, it's something to strive for. And the taper should really be fairly even all the way to the top, ending at a mere tiny stem at the tip.
Branches wouldn't have this kind of taper, but I would say that in the range of between 30 to 1 and 75 to 1 is pretty good. But you'll often see trees with almost no taper, and the branches might be 200 to 1! Pear trees commonly have nearly untapered branches, and this is one of their downsides. Do what you can to avoid this.
So for a good example, if you have an adolescent ornamental tree that's 20 feet tall, maybe the same width or maybe wider, the trunk should be 8 to 12" in diameter, and thicker as it gets old. A 3" branch coming off the trunk at about 6' off the ground should be from about 8' long to say 15' long, while keeping it all in proportion (height to width of the tree while tapering of branches thoughout). This takes some thought and planning and a good aesthetic sense.
Proper pruning will also allow you to control the direction better, and promoting more horizontal branching than vertical is usually better, even in fastigate or upright trees like poplars or liquidambers, some birches, and lots of conifers.
Of course, as you bring a tree home from the nursery, they're usually 6' to 10' tall, with a base diameter of about 3/4" to 1-1/2". Nobody's gonna wait for great tapering before putting it in the ground. They want to get the trees in the ground and on their way. And then they stake the tree so it won't fall over or curve too much, though staking weakens the tree, and in my opinion makes trees grow too perfectly straight. So it'll take at least 7 to 10 years in most cases before the trees get decent trunks and scaffold branches. As a side note, remove those stakes as early as you can. I've seen trees with stakes left in for 20 years or more, with the trunk growing around and devouring the big black rubber band. That REALLY shows that people paid no attention to their trees after the first couple years.
I'll keep working on this and see if I can get some better numbers and some pictures to demonstrate. In the meantime, give this some attention when you start to prune a tree. Strive for even tapering, never blunt tips of long, untapered branches. Ramification is so important.