Swelling of funnel bases would, I suggest, come about because of a number of factors. Situation of boilers IE two or more athwartships, size/type of boiler,breadth/ height, Scotch or Yarrow, multiple boiler rooms, space available to fit the boilers or combination of boilers, depth of hull in a long skinny vessel, and the need for a easy flow for the combustion gases. Support of long tall funnels, (furnace draught considerations) also needed a solution and a larger base would assist.
From the skippers point of view, windage would also have to be taken into consideration.
Bob Ferguson.
PS Can't spell athwartships. Had 3 goes and still not sure