I was watching some episodes of the Pokemon X/Y anime (or Pokemon XYZ as it's also known), and I noticed something about Serena's Sylveon, namely that it's unusually small for a Sylveon.
You see, the pokedex in the games says that Sylveon is 3'03 (99.1cm), you know which other pokemon is 3'03? None other than everyone's favorite furry magical girl, Braixen. As you can see from the image above, Serena's Braixen towers over her Sylveon.
Also, in case you're thinking "Maybe Serena's Braixen is just really tall for a Braixen", there are several episodes (and specials) where Serena's Braixen is seen standing near Clemont's Luxray; the games' pokedex says that Luxray is 4'07 (1.4m), and Clemont's Luxray is slightly taller than Serena's Braixen in all those episodes/specials, indicating that Serena's Braixen is (at least roughly) the correct height for a Braixen.
Now I'm not saying that Serena's Sylveon being small for a Sylveon is a bad thing or a plot hole or anything like that, just that it seems to be the case here. There have been other examples of members of a species of pokemon varying in size both in the anime and the games. For example, Ash's Charizard, who is apparently small compared to other Charizards. For an example from the games, there's Ride Pokemon in Pokemon Sun/Moon like the Mudsdale, who is much shorter than 8'02 (2.5m) and Machamp, who seems to be bigger than the average Machamp (which are 5'03 according to the 'dex) since you can find Machamps all over Alola and stand next to them showing that the player character is roughly the same height as a Machamp.
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