I just checked a stripped copy of postgresql-10 built with all options on and it is 7MB. That could be reduced a little by leaving out language support and ssl support etc.
Sqlite doesn't need too much more memory than what its binary needs whereas with postgresql, you need all sorts of bells and whistles just to get the database system to boot.
Anyway, not trying to make the case that postgresql is small compared to sqllite, it obviously isn't, just wanted to point out that it's not _that_ big either.
Is it possible to dynamic link system openssl to reduce size?
It is dynamically linked to openssl, but you can configure out the internal code to support the functionality.
"strip" however meant the unix strip command to remove debug symbols.