The bigger the wheel, the lower the rolling resistance (not the same as drag) but the greater the centripetal force that tries to pull the tyre off the rim. Modern aircraft have higher take-off and landing speeds than older, slower machines, necessitating wheels that can reach higher angular velocities - hence, smaller diameters. Not having to provide clearance for a prop helps as well.

To reduce contact pressure on the tyres, add more tyres:


Just one reason, maybe not the reason. It would also be cheaper to manufacture more smaller tyres than fewer larger ones.

BTW, hyia. New guy here.

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1995 W202 C250 Diesel, OM605 NA engine, 5-speed manual
1986 W123 200, OM617 NA engine, 5-speed manual, rev counter
1989 W126 300SE, 5-speed manual