[RedCrowRides]

The thing on a 125cc Bike is, you are dealing with very limited HP and Torque as a constant ,and it is a lot more apparent in a small displacement when changes are made that negatively impact dynamics. What i mean is , even a slightly heavier tire ( larger)weighs more ,and it's going to take that much more to drive it. Anything larger / heavier than the absolute minimum needed to maintain adhesion and traction is going to cause issues due to that - addtl drag / resistance due to weight and/or excessive contact patch.With todays amazing tire compounds which allow much better traction / adhesion than even the best tires of even a decade or so ago ,I'd focus on using the smallest ( lightest) tire sizes I could get away with , and balance that by emphasizing the compound selection for the type and style of riding i intended to do .
What I'm saying is a small race compound rear tire will have more "grip" than a much larger tire of a harder compound ,even though the larger tire has more contact area, it isn't going to grip as well due to the overly hard "over road" compound.



So, i tend to agree with Chinabike on 120/70 and 130/70 as tire sizes being pretty much optimal, I'd be more diligent on looking at the weights between tires ,and the compound more so than the size , you just aren't realistically likely to " out power" a small tire on a 125 , like perhaps you would on a Full Size sport bike, when tire sizing can become a much more critical factor.


Anyways that's my take on it ,filtered thru the lens of it's going on a 125cc machine ,it's entirely possible someone will come along and prove me totally wrong ,and/or call me insulting names ,neither of which will be a first time occurrence lol