When Josh grows up, he will try and find Susan to continue their relationship.
Susan went back to Zoltar and became the 'right' age and they met up in the nineties.
Mac-Millan used the Zoltar Machine 20 years prior to the events of Big.During the piano scene it seemed like he was not only asking Josh about his opinions on toys, but also studied his behavior like he recognized it before. Through Josh he suddenly remembered himself as a kid. Only someone with the mind and insight of a child could make better toys for the company, hence since Josh was promoted and Mac-Millan was already head of his company there is only one conclusion. The Machine itself looked like it was around before even Josh was born. This also gives a good reason why he quickly promoted Josh. He needed to watch over him to make sure his child like antics don't get him found out.