What shonky is saying is correct the larger displacement the engine has the more air the turbo must flow. So a 2.0litre yb engine will make 350-400bhp using a t34 as the turbo can sustain the air pressure required by the cosworths displacement. Therefore if you use a higher displacement engine ie 3.0litre the turbo now needs to flow more air to make up for the higher displacement.

Put it into lamens terms get an empty smarties tube and take the ends out, put one hand over one end and blow into the other as hard as you can. You should be nearly blowing your hand away from the tube. Now try it with a Pringles can and you will understand what we are talking about.