The documents icons are shown using css styles and a background image url and position pointing to the right icon in the document icons image sprite. I guess the small document icon is a result of a smaller element which holds the according css class for the icon (sftxt, sfpdf). I suggest checking the item parents in case the size is passed using percentage or relatively to the parent element and adding a specific class to those items, so you can set them the size you want.
I hope this information will help you resolve the issue.
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