Is the longer word count on an extended sample really a big deal?

Yes! Some issues don’t come up in the standard sample edit. For one thing, the first few pages of a manuscript tend to be the most polished. For another thing, some issues may not be present at all in that limited sample. Which is fine for a typical sample, but it doesn’t really give you solid examples you can apply on your own. And, finally, with a sample edit, you don’t really get a feel for the editor’s style and personality—there’s just not enough material for that to come through.