Painting Small

Years ago a teacher told me that many of his works, which were primarily for illustrating book covers and advertising, were done in quite small sizes--as small as 6x8 inches--before being photographed and used in larger formats. For a long time that approach didn't make sense to me, painting small and showing large, but it began to make sense as I did a quantity of small oil sketches. 

"Mountain Sketch," oil on panel, 6x8

Looking at the sketch above you might think it's a fairly large work, but it's only 6x8. There appear to me myriad details in the trees, particularly the two large spruces, but those marks are actually quite tiny. The distant mountain were painted very thinly and then mostly wiped off. The whole thing looks as it it could be at least medium-sized, say 12x16 or larger.