Big/WMG

When Josh grows up, he will try and find Susan to continue their relationship.

This was hinted at in the end. "Maybe you should hold onto my number..."

Susan went back to Zoltar and became the 'right' age and they met up in the nineties.

Susan reconsidered Josh's offer and used the Zoltar machine several years later (de-aged to teenager, maybe) to meet up in the nineties, teenage Josh will introduce her to his older parents as his girl 'friend' and his parents will be happy?

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.