The following answer is taken from Wardale's response to an unpublished letter from Bryan Attewell written in response to Wardale's two-part article "Steaming into the Future" published in Steam Railway magazines #272 and #273 (see 'Articles and Letters' page of this website):

"Mr Attewell would like a larger boiler, but what counts most for steam production is the effectiveness of the exhaust and combustion systems, and all a larger boiler would really do is spoil that most important parameter - vision ahead from the cab."