Head-cannon: Pokemon Genetics.
Eevee is one of the most versatile pokemon in the pokeverse . It has a chance to evolve into one of eight different evolutions.
It is in the field egg group. While an eevee still needs another eevee or eeveelution for its mother, it has 200 suitors [maybe less because of the 100% female gender ratio for some pokemon] that could be it’s father.
With that in mind I like to think that an eevee could learn egg moves from the right father, but it has a chance to carry physical traits as well. They wouldnt be as cookie-cutter as in the games and anime if that was happening. There’s a chance for different body types, colors and fur textures.
If that was the case, pokemon breeders would take that knowledge and breed eevees with a pre-determined plan for its evolution.
I dont think Sylveon would still be small and dainty if its father was a Granbull.
Pokemon Breeders could go other ways too. They could make designer eevees — eevees with other pokemon markings that would be used as pets. Or Eevees with strong parents from other types so when they evolve their special defense would go up when it comes to their weakness. [ Darmanitan-Dad Eevee. If it evolved to a Leafeon instead, Fire moves wouldnt hurt as bad because of its bloodline.]
I like to think all pokemon would look different in some way or another if we took in mind which parents we leave at the daycare.
I mean, it’d be awesome to have Shinx who learned Thunder Fang from his Arcanine dad AND got his cool stripes.