Players in Pokémon GO may have the chance to look forward to enlisting their Pokémon back into Pokémon Contents to potentially receive rewards for entering the largest Pokémon.
Pokémon Contests are a regularly returning feature of the main-series games. Throughout the series, from Generation II onwards, some iteration of the Pokémon Contest has been present. Whether this exhibited itself as the Bug-Catching Contest of the Jhoto games, or the varying Contest competitions players could go in Sinnoh games, there typically is some Pokémon Contest players could compete in as an additional feature.
Specifically in Pokémon HeartGold and SoulSilver, players could catch Magikarp and bring it to a specific fisherman for him to tell the player the size of their Magikarp, and catching larger ones would illicit differing reactions from him. Allegedly, something similar may be coming to Pokémon GO.
Pokemon Contests Leaked to Return to Pokemon GO
Allegedly, Contests will be coming to Pokémon GO in the sense that players will be able to compete by "comparing size[s]" of their Pokémon. This information comes from notable Pokémon GO dataminers, specifically, Twitter user @poke_miners.
So, will the Contests in Pokémon GO be similar to the aforementioned ones from the main-series games? Meaning, can we expect that players can face against other trainers looking to see who has caught the biggest Pokémon? Will these contests be species-specific or something much broader?
Only time will tell as to the legitimacy of this claim, but this user does not typically prove to be wrong after publishing their tweets.
Once official information is released on this subject, you'll be able to find about it here, on DBLTAP.