Can anyone point me to the thread where its explained why 25mm tires have less RR than 23? Thanks!

Too lazy to find a thread but that statement is generally only true when inflated to the same pressure.

Less deformation.

Don't have a link to discussion of this, but basically most rolling resistance of a tire is related to the energy wasted in deforming/reforming the tread as the tire rolls. When weight (bike + rider) is put on the bike the tire tread deforms against the pavement. At the same pressure, contact patch in direction of travel is smaller for 25 vs 23mm tires. Smaller contact area = less tread deformation = less rolling resistance.
This is assuming all else is equal between the 23 and 25m tires (same construction, etc.).