What features of Oracle make it an attractive choice for small projects?
I can only attempt to answer for Oracle and postgres. After using Oracle exclusively for years, and postgres only for the last two years or so, I love postgres. There are so many small ways it is more convenient to use than Oracle, and it shares many of the crucial benefits (such as MVCC). It is easier to administer, reliable, has excellent documentation, and of course, it is free.

However postgres is no match for Oracle in some areas, such as:


RAC - as far as I know there is no better clustering technology for any database
RMAN - far superior to postgres's basic backup and recovery features, especially with block change tracking and incremental backups (which you can apply to other backups to keep an up to date full backup)
Oracle support - exists. postgres support? Not so much
many other features such as IOTs, 'Secure Files' and compression technology that have no analogue in the postgres world


Interestingly almost all af these features are absent or crippled in XE. I think I'd choose postgres over XE, all else being equal, but...

...none of this addresses the two biggest reasons for choosing Oracle:


You are already using Oracle and you have a huge investment in it (database independence is a silly myth)
Your developers and DBAs know Oracle and use its features to the full (why wouldn't they as database independence is a silly myth anyway?)


EDIT:

The one situation where I'd choose postgres over XE every time is if security is a concern. If your database or any of it's applications are exposed to the public internet I am not sure if XE is a good idea at all.