Can confirm that they do, since I tried that on our old dogs. The male was happy enough to work hard, including 50km marches where he carried his own food (dried ox liver), but when practicing commands and such, he performed worse the smaller the treat got. When I increased the size, he immediately was on his best behaviour again. His female mate was a case study in too smart: when training her to fetch by throwing a stick - you know, the normal way to do it - for her to run off and find in the underbrush, she'd simply saunter off to the closest stick and fetch that one instead, looking smug as F.