The increase in permissible spans depends upon the loading situation and the relevant wood parameter that is constraining the situation. Usually, the later is either bending (expressed as a fraction of total length -- like 1/360) or extreme fiber stress (Fb). The numbers I mentioned above are for the condition of uniform loading and total bending. Thus, for the two loading conditions the increases in span would be 25% or about 40%(1.25**3/2) with the 25% increase in thickness.
