It depends on what is installed for each user. You son may have additional
features installed for his user profile (e.g., games, office apps,
third-party apps, ...)
The basic ... new user size is 280-512KB.
And how much cleanup has your son done on his account ... if you are basing
the size on the Properties of C:\Documents and Settings\[USERNAME] folder,
that can include junk files as well as settings and valid Application Data.
And are you both the same type of user ... Administrative or Limited? Each
account type has different initial sizes as well.