It's the base stats that determine it, usually.
>Base stats are the general outline of statistics for a specific Pokémon species.
Base stats usually give a general idea of the strengths and weaknesses a specific Pokémon will have. Pokémon often focus on some stats more than others. For example, Umbreon is mainly a defensive Pokémon; therefore its base stats in Attack and Special Attack are low compared to their corresponding defensive stats.
Base stats range from 1 to 255, and are most often the prime representation a Pokémon species has in battle. For example, Blissey has the highest possible HP base stat (255), but has Attack and Defense base stats of 10.
Commonly, related Pokémon will have similar base stats, though distributed differently. This is one main difference between the two or more branches of a split evolutionary line. For example, while Gardevoir's base stats for Attack and Special Attack are 65 and 125, respectively, its counterpart Gallade's Attack and Special Attack stats have inverted values. Pokémon of a higher evolutionary stage will also usually have higher base stats than those of a lower one, and will have higher total stats than those of their pre-evolved counterparts; the only exceptions are Shedinja, whose base stat total is lower than its pre-evolved form, Nincada, by 30, and Scizor, whose base stat total is the same as its pre-evolved form, Scyther.
Basically, the higher the base stat, the higher the stat usually is. For example, Breloom has a 130 base attack stat and a base 60 SAtk stat, meaning its Attack will usually be higher.