Does it matter that the second volume of my two novel series is about 20,000 words longer than the first?
No. But something to consider is that publisher don't usually take on a series written by an unknown author. So make sure the first volume can stand on its own to maximize your chances of publication.
That's true, but what I was getting at was that your first novel shouldn't have any major cliffhangers, and readers shouldn't need the second novel to understand what was happening in the first. For example, consider the first Harry Potter book. All major conflicts are resolved, and there really aren't any loose ends to tie up. So the first book stood on its own, and the second book picked up another major conflict.
Oh, that's what I meant. The first book comes full circle. I didn't think to continue it until after several drafts. And it's a story told in vignettes, so there's a lot of room to play with the character and overall plot arc.
From what I have seen, the subsequent books in series are generally longer than the first for many reason, one people that if it's an unknown author, publishers and agents like to keep the word count down on the first book and then they can bulk it up in the other books. Also as another poster mention, I think its best that your first book for through the entire editing process first prior to starting the second book because you never know what may change.
Actually, at a writer's conference over the summer, Paula Munier of Talcott Notch Literary said that agents are warming to the idea of a series, and having a book be the possible beginning of a series is now a point in its favor. However, @thirdwind is correct - the first novel must absolutely stand on its own.