What a stated size leaves out: tails, tentacles, horns and bills
Every measurement is a decision about where to stop. On a machine that decision is written into a drawing. On an animal it is a convention, and the convention usually excludes the most memorable part of the animal.
The clearest case: the giant squid
The catalogue holds a giant squid at 4 m. Anyone who has seen a photograph of one laid out on a dock will think that is far too small, and they are right about the photograph and wrong about the number.
Squid are measured standard length: the mantle and head, stopping before the arms. The two long feeding tentacles, which are the reason the animal looks the way it does in every picture, are excluded because they are elastic. They stretch in life and contract after death, so their length is not a property of the animal in the way a mantle is. Include them and the same squid becomes roughly ten metres. Both figures describe one animal and only one of them is repeatable.
This is why a cricket pitch at 20.12 m against a giant squid at 4 m gives a ratio of 5.03 rather than the two you would get from the tentacle figure. The comparison is honest and it is worth saying what it excludes, which is exactly what this kind of note is for.
Horns, antlers and tusks
These are excluded almost universally, and for the same reason: they vary with age, sex and individual far more than the body does.
A white rhinoceros at 3.7 m is a head-and-body figure with the horn left out. The front horn can add most of a metre and is keratin, the same material as hair, and it grows throughout life. Set the rhino against a manta ray at 5 m and the ratio is 1.35, which would move to about 1.06 if you counted the horn. One excluded feature, most of a third of the answer.
Moose are worse. The figure used here is 3.19 m nose to tail, and a mature bull's antlers span close to two metres on their own, entirely outside that number. A moose at 3.19 m against a thoroughbred at 2.4 m gives 1.33, and nobody standing in front of both animals would guess a ratio that low, because the antlers are doing all the visual work.
Woolly mammoths are quoted at the shoulder, 3 m, with tusks that could curve more than four metres sitting outside the figure altogether. Against an emperor penguin at 0.95 m the ratio is 3.16, and the tusks are longer than the entire penguin.
The rule of thumb: if a feature grows for the animal's whole life, varies between the sexes, or bends, it is probably excluded from the stated size. Horns, antlers, tusks, tails on some species, tentacles, manes and crests all fall under at least one of those.
Tails go both ways
Tails are the inconsistent case, and it is worth knowing which way each group falls.
Included: reptiles, where total length is the standard. A saltwater crocodile at 4.5 m and a Komodo dragon at 2.4 m both have roughly half their stated length in tail. Dinosaurs follow the same rule, so Diplodocus at 25 m against a blue whale at 27 m is comparing a figure that is nearly half tail against one that is mostly body.
Excluded: big cats, primates, capybaras and most rodents, which are quoted head and body. A tarsier at 0.28 m has a tail longer than its measured body. A capybara at 1.2 m against that tarsier gives 4.29, using two head-and-body figures, which is at least consistent.
Depends: dogs, where breed standards use the shoulder and popular sources use nose to tail. The miniature dachshund at 0.38 m is a nose-to-tail figure, which is unusual for a dog and matters for any comparison that puts it beside a breed quoted at the shoulder.
Bills, beaks and the toucan problem
A toco toucan is 0.6 m measured bill tip to tail tip. The bill alone is roughly a third of that and is almost hollow, a lattice of bone with a keratin shell, which is why a bird that looks front-heavy is not.
Here the convention includes the feature rather than excluding it, because a toucan's bill is fixed in size once the bird is adult and is not going to keep growing. A tennis racket at 0.685 m against that toucan is a fair comparison of two objects measured end to end, and both of them are mostly air.
Machines have the same problem, written down
The difference with built objects is that the exclusions are documented rather than conventional. An Airbus A380 is 72.72 m long, and the wings, which reach 79.75 m tip to tip, sit outside that figure entirely. The catalogue keeps a plan view of the same aircraft as a separate object, because the outline you see from above is not the outline you see from the side. The figure attached to it does not change: it is the same 72.72 m, and the wings still fall outside it.
A Boeing 737 is 39.5 m along the fuselage and about 12.6 m tall at the tail. Against Elizabeth Tower at 96 m the stated ratio of 2.43 uses the length, which is the conventional figure for an aircraft, and comparing a length to a height is the caveat that has to travel with it.
The question to ask
When you meet a size in print, one question does most of the work: what does this number stop at? For an animal it is usually a joint or a notch. For a building it is a roof, a spire or an antenna. For a ship it is a bow and a stern. For a country it is a coastline and a compass direction.
Every comparison page on this site answers that question in a column of its table before it gives a ratio, because a ratio built on two unstated conventions is not a measurement. It is a coincidence.