In 0.6.36.138, it looks like the length calculation doesn't respect the diameter of SRBs anymore, thus boosters with small diameters are too long and have too low thrust to push that higher weight.

Interesting... must have broken some of the volume/thrust calculation code when working on the upper-stage SRBs. Looking into this now, will let you know what I find out.

Ahh, this actually appears to be due to a miscalculation when setting up the model scales; at some diameters they get stretched vertically, but the thrust calculation is not including this volume. Testing out a fix now, and will see where this leads.

Seems to be working properly again, thanks for pointing this out.