To introduce a little more technicality than neox did (if it helps get the point across), the critical quantity with regards to airflow is the mass of air you're pumping into the engine (how many air molecules).
Generally, a smaller turbo is less efficient at flowing large volumes of air, and this loss of efficiency means that the air is heated more as it's compressed. As we all know, warm air is less dense than cool air. This is true at atmospheric pressure (why we use CAIs on N/A motors) and also on turbo cars--since warm air at 10psi is less dense than cool air at 10psi.
Thus, a larger turbo is generally going to flow a larger mass of air at a given pressure as a smaller turbo at the same pressure, and why you can't necessarily equate 10psi on a gt28 (for example) to 10psi on the stock turbo. Another reason you can't directly compare them, as far as safety goes, is that since the 10 psi charge from the small turbo is hotter, your chances of predetonation start to increase. Since the larger turbo delivers air at a cooler temperature, you may be able to flow a higher mass of air safely, meaning more safe power--depending on the particular motor's weak points.
Disclaimer: I'm by no means an expert on turbos... just going off what I've learned over the past year or so, and an engineering background.
