On a road bike, I put a larger tire in the rear (say 25mm front, 28mm rear). This is bvecause the rear carries more load, does not have as high cornering forces, and mostly becasue I want a softer ride under my butt.
On a gravel bike, I put a larger (and more aggressive) tire on the front because
- I'm not so concerned about high speed aerodynamics
- On really tough trails/roads, the front is going to loose traction first with tires around 35-40mm.
- I want more shock absorption in the front for the rougher stuff.
- I don't want tread or carcas distorition from high torque in the rear tire - on the front tire, distoriton only matters for braking and cornering.
Really, on dirt, it is mostly about traction in the front tire.