In the preview video for USJ, Mario is about half as tall as many of the adults in the video. If a cast member is wearing a suit the close to the size of an average adult like one seen at the groundbreaking ceremony, an explanation could be that Mario is about the size as he is in the preview video without taking a growth mushroom and the size of the suit character after taking a growth mushroom. I guess they could even have huge Marios in parade floats or as giant animatronics or marionettes for something and have the explanation be that Mario is that size after taking one of those super growth mushrooms like seen in the recent 'New Super Mario Bros' games. That could even be used as the explanation for a of characters even Pokemon if they allowed crossovers of things to occur in the parks.