I'm currently outlining one of my stories to be. It's pretty lengthy already (It could easily qualify as a novella from the length of the outline alone). There are three volumes in total. The first volume is, more or less, fine. I have enough substance to keep it from getting completely stale. It's volume two and three that the events and plot points begin to strain. Volume one had twelve chapters in one part, with an average of seven chapters. Now I'm barely getting to four. I think the highest I had in volume three was six chapters. What's worse is that chapters in these last two volumes can even be combined and the action and pacing would fare better. If I did go through with that then I would eventually force myself in a corner as my ideas are being spread too thin. Any advice would be helpful.
My advice is to simply make two volumes instead of three. Combine two and three together. Or, if the plot doesn't allow it, then combine the chapters that you feel need combining in books 2 & 3 and throw consistency out the window. I think getting the pacing right is much more important than getting even volumes.
Personally i'd write the first volume first and worry about 2 &3 after that, as you are pretty much bound to kick off new ideas during the writing process. Outlining is suposed to be an aid not a straitjacket
Why are there three volumes when you don't have enough material to fill them? Don't stretch and pad your story just because you think three volumes look nice on a shelf. You have one story's worth of material, so write it in one volume.
What moose said. If you haven't actually written volume one yet there is no possibility of a volume two or three or ten. They can't exist by definition. That's like trying to buy clothes for your third child when your first hasn't been born yet.
I'm in the same boat as you - I've got way more material than I can reasonably cram into one book, but I'm not sure how many it will end up being. I think about it from time to time, but when I do, I just force myself to shrug and say "Whatever. It'll be as long as it is." When it's done, then I'll worry about adjusting it to fit into 2 or 3 or 4 books, because then I'll be able to see what I have to work with. Short version: Don't put the cart before the horse.
