It’s like confiming that something is built according to plan or to the engineer’s specs, can’t do it. I have no problem with a purlin which is smaller than the rafters. A 2x8 or 2x10 purlin under 2x10 or 2x12 rafters is fine as long as it’s adequately braced.
I like to see 2x6 purlin braced every 4 ft. It really depends on the rafter span.
2x4 purlins are common in older homes around here and they aren’t great but they’re a lot better than nothing and that’s what I tell people verbally but I don’t mention them in the report unless they have visible failure. If 2x4 purlin has held up with no failure for 50 years, I’m not going to call it out as being in need of repair.
I call it out as inadequate if I see signs of failure, like sagging rafters or purlin.