well, the A/R ratio you see is the cross sectional area vs the radius of the compressor housing. A = area and R = radius.
the bigger this figure is, the cooler the boost produced will be relative to another housing of a lower A/R ratio.
for example, a 0.7 A/R compressor has a greater "area vs radius" calculation than does a 0.5 A/R compressor. now, if both the radii are the same for each compressor, then that means the one with the smaller A/R has a thinner compressor housing and will flow less outright than the larger A/R.
if the two compressors are off the exact same turbo (same wheels, same cores and same exhaust housings) then the one with the larger A/R compressor housing will produce cooler boost because although the pressure is there, it isn't being "compressed as hard" - for lack of a better term - due to there being more room inside the housing.
remember, boost isn't only what air the turbo can compress, it's also got to do with what prssure the engine can consume - ie a T25 compressor housing on a CA18 might make 9psi std with an exhaust housing to suit a 1.8L. put this same trim compressor on an A12 with an exhaust housing to match a 1.2L and it might make 14psi. same compressor, different results.
the best way to size a turbo is to look at the air your engine can consume, and then work backwards and decide on the exhaust housing (this will improve / kill spool up response), choose your core (plain bearing or roller bearing), choose your impellers and then choose your A/R for your compressor housing to match all of these put together.
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