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You said it's not surging during operation and yet the level is dropping 3"? That is surging. Cue "Black Water" by the Doobie Brothers

The surging water might not be getting all the way up to the radiator, but the sound of it might be going there. It might only be surging up to the header (which it is a nice header, by the way)

Agree, if indeed there is more vertical space and we would have to assume water of same purity shouldn't boil any more violently (vertical displacement) in a bigger boiler than a smaller one then this could be some of the explanation.

@dabrakeman

The closer the top of the water is to the steam tappings, the more likely it'll be to pull water up into them.

I could only guess the larger boilers have more distance between the top of the block and the normal water level.

Check the piping for pitch. Maybe something changed with the boiler swap. Is the rad hooked up 1 pipe or two pipe?

Did they skim the boiler and is the water level steady while steaming?

Need more pics of the boiler piping.

5 section boiler calls for 2 2" risers or 1 3" riser. As others have mentioned its piped wrong but the fix is easy.

Remove the 2" riser that is between the building supply and the equalizer and cap the boiler and the tee.

Remove the other 2" riser and install a 3" riser. Easy to go back to the flange and make it a three elbow swing joint with 3 90s

and @EdTheHeaterMan even patched the hole in the boiler jacket!!

It says 2 weeks, no?