The correct answer (note the brackets):
(//span[@class="Big"])[1]
The following expression is wrong in the general case:
//span[@class="Big"][1]
because it selects every span element in the document, that satisfies the condition in the first predicate, and that is the first such child of its parent — there can be many such elements in an XML document and all of them will be selected.
For more detailed explanation see: https://stackoverflow.com/a/5818966/36305