Thank you for your question.  I have looked at your picture and am unable to determine where on the tree these leaves are located.  So, I am not entirely confident of my response.  Below is a link that discusses why the size of the leaves on the same tree is not uniform.  The size varies because of light.  See the entire article for all the details.
http://northernwoodlands.org/articles/article/are-all-tree-leaves-the-same

They are located on a branch about a third of the way up the tree. The branch otherwise appears to be mostly dead. 

I am not sure if you have another question. The information in the link I sent previously indicates that shade leaves grow on the lower branches. I had also consulted a reference frequently used by CO Master Gardeners (Insects and Diseases of Woody Plants of the Central Rockies) and could not find any information about differences in leaf size. So, I think the explanation of sun and shade leaves answers your initial question.

Since your believe the branch is "mostly dead" I am providing a link to tree pruning that provides excellent details on why and when to prune. Perhaps you should consider pruning out this branch.

http://www.ext.colostate.edu/mg/Gardennotes/615.pdf

I received some very good additional information about your question. See below.

Giant leaves may be due to dieback of twigs/branches of the aspen. Had these leafed out normally, those new growth shoots produce hormones (auxins) that inhibit reserve buds in the trunk and larger branches. Since those twig ends are dead, there are no auxins to inhibit the latent or reserve buds ("emergency buds") produced by the tree in earlier years. So these reserve buds sprout out, because the tree is truly facing an emergency. Reserve buds produce new juvenile shoots with l a r g e leaves to capture sunlight and make some photosynthetic foods for the tree. So, necessity, juvenility and hormones are the reasons for these large leaves. 

Thank you to Robert Cox for taking the time to provide these details. 

Thank you Robert!
That makes a lot of sense. Mother Nature in survival mode!