Cuando fui mayor

Not at all. Similarities happens only with the fantasy element and the child x adult romantic pairing. The premise of both movies is the same but the characters and solutions to all the conflicts are completely different. It's not a remake in disguise.

The Italian movie has a very neat script that not only is funnier but is more clever in comparison. Big has a corporate play that dominates the movie while also putting the parents away for the sake of the plot. Da Grande makes intelligent use of the parents and the kid's teacher, the love interest. The age gap of the main character is larger, a 40 year-old behaving like a 8-year old that acts more like a absent-minded child than Tom Hanks character so there is more laughs. The script makes great use of 'child logic' in an adult setting.

Big is an iconic movie for the 80s and a key moment for Hanks' career, albeit it is a bit laboured and slow. I found Da Grande funnier, faster-paced and especially more sweet. Too bad it's hard to watch it outside Italy.