This is the total output of each SRB found in game at 100% burn rate

In this table we can see the total thrust (Max load, in kg). This is where we find the first irregularity with SRBs. You would expect the power to go up directly in response to the size yet, that is incorrect.
For example, the small SRB is 48 times smaller than the medium yet the medium only outputs 8.5 times more in thrust. But this is 1 of many irregularities found with the SRBs.

Next we find the total burn time without any extra fuel. You can easily see that the small SRBs is the only one that doesn't fall into the 2.5-2.7 burn time.

After that we have total thrust output to how many blocks is used up by the SRB. Here you see that the small SRB leaves the larger ones in the dust and with good reason. The small burns all of its fuel 2.5 times faster than the rest! At first I didn't believe it so i ran another test between the medium booster and the small to see who can out push the other
and the victor was the small by a long shot. I fixed this by either increasing the larger SRBs' burn rate or by decreasing the smaller SRB's burn rate to make it soo that every SRB burned for the exact same amount of time. That is where I got the "corrected" numbers. You can still see that the smaller SRB stays at the top for total output but oddly the medium is the worst out of the 4.

I used this contraption to measure the max weight a SRB can carry