There was some discussions about this so I found and easy way to reproduce.
I am creating a poly feature class in Sql Server (10.5) with Israel TM grid projection (WKID 2039).
I am creating a CIRCLE polygon. Then I copy & paste the layer into FGDB. There is a different in the area (see picture). The different in small polygon (400 Sq Meter) can be up to 0.5% (this is a lot).
Copy into Oracle give very similar area as the SqlServer.
The one that has no difference is a square, all other are circles.
I did not tested different projections.
Take 3 minutes and try to see for yourself.