Effect of flour/fat/sugar/egg ratio in a sponge pudding
Michael Ruhlman talks about cooking in terms of ratios in his book: Ratio: The Simple Codes Behind the Craft of Everyday Cooking.

He says the basic Sponge Cake (close cousin of Sponge Pudding) is (ratios by weight):


1 part egg
1 part sugar
1 part flour
1 part butter


And it works. This BBC recipe is along those lines.

To your Q on ratios, to make or more dense cake you could:


decrease an egg
and/or increase flour


You have other options as well. 


you could reduce leavening agents, if any
at the point you are combining butter into another ingredient, be careful not to over do it, this introduces more air into the final product.