"Moil" means to work hard; it is a word not heard often nowadays outside of the expression "toil and moil." Stevenson is saying that these "folk" are not interested in any activity that is not productive; that for them an hour not spent doing something lucrative, something to earn money, is an hour wasted.
"A man's soul were not too small to begin with..." We humans are already restricted by our nature. We must necessarily concern ourselves with the mundane and can obtain only occasional glimpses of the Divine, if I can put it that way. Instead of attempting to broaden themselves by contemplation of beauty, listening to music, discussing the arts, philosophy, etc., these money-grubbing people are making their "souls" smaller by focussing only on work.
Does that help? I hope I haven't made things worse. :)
[native speaker, not a teacher]