The whole topic of scale effect is highly subjective, so I'll skip the debate and refer you to an respected source:(http://www.ipmsstockholm.org/colorcharts/stuff_eng_colorcharts_scaleeffect.htm)
That site, by the way, of IPMS-Stockholm, is just loaded with excellent information on colors and paints.
For what it's worth, my (subjective, of course) opinion is that the XtraColor paints look scaled-down for 1/72 right out of the bottle.