Jerry
I note a discussion on another aviation website about putting on larger aircraft on the Cardiff-Valley route and that it has been pointed out that Valley is currently restricted in the number of passengers it can handle. That may be the current situation but as long ago as July 2015 (Public Accounts Committee report) the Welsh Government was discussing the idea of a larger aircraft in the wake of the abolition of APD on PSO routes.
It was suggested then that to upgrade facilities at Valley to take larger aircraft would probably cost in the region of one million pounds which is why I said that it would be expensive to use larger aircraft.
The below link is to the Public Accounts Committee report - read from para 44 onwards.
http://www.assembly.wales/laid documents/cr-ld10318/cr-ld10318-e.pdf