This remains a mystery
There is no information about this on Bulbapedia.
However I checked another question here on PokeDb and I found an answer made by TrollPatrol
If you have the big Pokedex book for HG/SS, it will show you the differences of Magikarp sprites. The differences of spites are very small. I emphasize very. Yet the main feature to tell the size of a Magikarp is something to do with their tails, either the shape or size of the encountered Magikarp. Unfortunately, I lost my copy and can't tell what is the actual shape or to verify.
This has to be verified though.
Some people think that the size of a Magikarp depends on it's IVs, which also needs to be verified.
I found a thread in a forum which speaks about a formula of finding the size of a Heracross. If you want to check that out click HERE!
- Do Pokemon have fixed sizes?
In the games they probably do. Maybe Magikarp is an exception. (Just my opinion)
In the anime there are many Pokemon with size discrepancies. Ash's Pokemon are usually smaller than other Pokemon of the same species. This is considered normal in the anime.