When a character is redrawn, the size changes
A character with no body has no fixed size, only a most recent one. That is fine for a story and awkward for a comparison, because the figure you quote is really the figure from one particular film.
Godzilla, which has grown by a factor of two
The figure in this catalogue is 119.8 m, and it is the 2021 film design. The original 1954 creature was around fifty metres, a height chosen so that the animal would look right against the Tokyo skyline of the day.
That is the whole mechanism. The character is sized against its setting, the setting got taller, and the character followed. Nothing in the fiction required it and nobody objected, because a monster's height is a dramatic property rather than a physical one.
The consequence for any comparison is that the figure is only meaningful with a year attached. Against the Burj Khalifa at 828 m the 2021 design gives 6.91. The 1954 design would give something closer to sixteen and a half, against a building that did not exist.
King Kong, which grew further
The same story, more extreme. The 1933 Kong was a stop-motion model on a table, sized for a New York skyline and an ape that could hold a person in one hand. The 2021 design is 102 m.
Godzilla at 119.8 m against King Kong at 102 m gives 1.17, and that ratio exists only because the two characters were designed for the same film and therefore had to be sized against each other. It is one of very few invented comparisons where both numbers were fixed at the same time by the same people, which makes it unusually trustworthy.
Against the Empire State Building at 443.2 m the 2021 Kong gives 4.35. The 1933 version climbing the same building was a very different image, and that image is what most people still carry.
The rule this suggests: when two invented characters are sized within a single production, their ratio is reliable. When they come from different productions, the ratio is an accident of two independent decisions.
Robots, and the design that changes per film
Optimus Prime is 8.5 m in this catalogue, which is the live-action film design. The animated original was quoted far smaller, and the two have almost nothing in common at scale. Megatron at 10.7 m has the same problem and has changed between films within the same series.
Optimus Prime against a gorilla at 1.3 m gives 6.54, and an AT-AT walker at 22.5 m against Optimus Prime gives 2.65. That second one crosses two separate franchises, which means two unrelated design decisions made decades apart, and the ratio is arithmetic rather than anything a story ever intended.
Which is fine, as long as it is said. The comparison is still a comparison of two published figures. It just is not a comparison of two things that were ever meant to be seen together.
Ships, which get revised upward
Fictional spacecraft have a habit of growing between sources, usually because a later work needs the interior to contain more.
The Millennium Falcon is 34.75 m nose to tail, and that figure has been revised upward twice in official sources. The refit Enterprise is 305 m, and the original television design was stated shorter, a discrepancy that has been argued over for half a century.
The Enterprise against the Falcon gives 8.78, two ships from two franchises, each with its own revision history. The Falcon against a TIE Fighter at 8.8 m is 3.95 and much safer, because both figures come from the same design lineage and were fixed against each other.
Why nobody minds
The interesting question is why a character growing by a factor of two provokes no reaction. The answer is that size in fiction is relational rather than absolute: what matters is that the monster is bigger than the buildings, that the robot is bigger than the truck, that the dragon fills the hall.
Smaug is a good case. The film design is 110 m long, which against a 105 m football pitch gives 1.05. The book gives no size at all, describing only that the dragon fills the hall it lies in. That description is dramatically complete and numerically empty, and for the purposes of the story it is entirely sufficient.
The number only becomes necessary when someone wants to draw the creature beside something else, which is precisely what this site does. So the figures exist, they come from designs, and the design has a date.
What this catalogue does
One figure per object, from one named source, with the version stated. Where a character has been sized several times, the most recent widely used design is the one recorded, because that is the one a reader is most likely to be picturing.
The alternative, recording a range, would be more complete and unusable. A game cannot score a guess against a range. And a comparison page cannot give a ratio when one side is a span of values, only a span of ratios, which is a much less useful thing to read.
So: one number, one source, one date. And a note on the page saying that the number is a design decision rather than a measurement, because that is the difference that matters and it costs a sentence to say.