[lorenzhs]

You’re wrong, the sizes of both input files are commonly considered when analysing algorithms that work on two files. Consider a problem that could be solved by sorting one of the files and then stepping through both files item by item (not saying that any meaningful problem with this property exists, it’s just an example). Then you would have a complexity of O(n log n + m) assuming n is the size of the file being sorted.

There is valuable information in running time being linear in the size of both files. You could define n as the combined size if you wanted to, but that could be confusing.