question on comparing emissions numbers

[DamageX]

I have seen results of emissions testing posted online using both parts-per-million and grams-per-mile (or grams-per-kilometer). It seems to me that these numbers would not be directly comparable. Using grams-per-mile, a larger engine/vehicle would tend to emit more than a smaller one. Whereas going by PPM, a large engine could very well score lower numbers compared to a smaller engine, despite emitting more total pollutants in practice.

Although, I'm not sure exactly what routine they go through to test emissions. I imagine that it is at most a simulation of driving a certain distance. The "per-mile" label could be a misnomer.

But I have seen Aus. and Euro standards quoted as being in mass/distance units, and at least one US state, eg.:

Aus. (grams per mile)
'76 3.36 38.9 3.06
'86 1.49 15.0 3.06
'99 0.42 3.38 1.01
'04 ---- 3.54 ---- HC+NOx=0.80 (Euro 2)
'06 0.32 3.70 0.24 (Euro 3)
'10 0.16 1.61 0.13 (Euro 4)

Maryland (grams per mile)
'00 0.70 15.0 1.80

Is it possible to tell whether a vehicle that meets one standard would meet another, or is the testing methodology simply incompatible in some cases? I ask mainly out of curiosity, because emissions aren't tested where I live. But sometimes the LEV/ULEV/etc. ratings are advertised, and it would be nice to know what these actually mean and how they compare to everything else.