How do you interpret compressor diameter, with reference to trim?
 

Ok - I couldn't find a better way of explaining in the title - so here goes!

Take the T25 turbo:
By my reckoning, there are around seven, maybe eight major diameters to these - ie the outer (larger) diameter.

In between all these, there are countless minor (smaller) diameters.

So, we have a T25 50 trim (for example) compressor map that we've found on the interweb or wherever.

How do we actually know if this was generated with a 56.00mm / 39.60mm compressor, or a 52.00mm / 36.30mm compressor?

It's gotta make a difference to the actual flow I guess... Anyone have any real answers?