Base stats, as known as species stats is hidden stats and indicates the three of the following:
Attack (associated with CP)
Defense (associated with CP)
Stamina (associated with HP)
Pokemon's CP is based on two of base stats, attack and defense. HP is decided by stamina.
Attack & CP Dragonite has the highest attack base stats at 250. That means Dragonite's CP is possible to be higher than that of other species.
Stamina & HP Chansey has the highest stamina at 500, and so its HP can become very high. However, its attack and defense is low relative to other species. As a result Chansey gets lower CP relative to other species.