When you compare the 125gr SWC, to the 148gr WC, look at the difference in case volume under the loaded bullet.

The 148gr WC seated about flush with the case mouth, leaves a small volume for powder. A small combustion chamber, which works great with the small charges.

The 125gr SWC leaves a lot more space for a small charge to combust in. It can work, but the 148gr WC seated flush, is tailor-made to optimize a small powder charge.