How does a panel get its current rating?

The panel may look identical to you but the thickness of the buss and the material makes a big difference where a inch of copper can handle 1000 amps per square inch verses aluminum of only 700 amps (NEC 366.23) so the buss material makes a difference it may look the same even be the same thickness but one being solid copper and one being aluminum with a plated copper makes a 30% difference. So a material difference or thickness, even the insulation used can affect the rating but we can’t really tell all of this with a casual observation.

The main breaker doesn't state the current of the busways.

Last year we had a run on questions about 20-space panels with 100A main breakers. The people wanted to add 25-40A of solar, and were getting different answers from solar installers. (With solar, you add main breaker + solar breaker... that can't exceed 120% of bus capacity.) It turned out their panels all had 125A buses, allowing 150A of main+solar. They were all set.

So you simply can't go by main breaker size as a gauge for busway capacity. You have to look at the specs and the label.

It's certainly possible for a panel of almost any size to be built for 125A or even 225A. As Ed Beal says, it's a matter of the busway design. There are a few 125A/30-space panels out there, and they also make 12-space 200A panels.