Surely it's not the size of the plane that matters - it's whether a service makes money. That is what makes a route sustainable!
A statement was made earlier that the Belfast route only had 60 odd passengers average but if they were business passengers who booked late and paid higher fares then the route might still make a profit.
Don't assume that low loads and small planes necessarily means bad news all round! With the proposed changes in aviation tax from November 2009 smaller planes with little or no empty seats might be a better way to go!!