This year’s April Fools’ Day joke on 4chan (and therefore on /mlp/ as well) was an arrangement in which users were placed onto different “teams” based off of IP-address.
 

 
As shown in the example above (from an unrelated discussion), it didn’t take long for a meta to start evolving around feelings of superiority or antagonism between different teams. (Team Peanutbutter was one team).
 
Since one of the teams was called “Team Mini”, a size-related joke about this was fairly natural to see made in the “My Massive Pony” thread.
 
/historylesson