Yvotcha, this one's for you:
You mentioned that the sections you carry are: Contacts/Notes/Information/Projects/To Do/Diary.
Diary + To Do:
You say you are currently using the DPP format, but will most likely have to revert to the W2P format due to lack of space.
You carry all your year with you because you need it for referencing, right?
From what I understood, the DPP format also allows you to write down your daily ToDo's? which is why a W2P would be too small...
From today start using the W2P format (I counted and it's only 28 sheets from today until end of year :P).
Then, take your To Do sheets and cut them in half. Keep them all neatly stacked at home, but have some 6 resulting sheets with you.
Place one half of the TD between your week spread, with the To Do's that belong to that week, should you need more, you can always add more halves.
Like many other suggested on your post, I also think you should store older months in another binder. Really think about it, which months do you still need to revise regularly? You could carry the last 3 months in a separate binder (maybe even a Slimline), if you REALLY need them. Older pages you can store in another binder at home, but keep it somewhere easy to reach, say your bedside table? That way if you ever need it to bring it to work with you, you can easily grab it and not spend all day rummaging through all your Filo parafernalia...
I am assuming this is your personal info. Maybe you carry all those Weight and measures etc....Think about it, how badly do you need it? Here you could just carry your personal details (one sheet) plus the Registration page with you.
If you're like most people, you probably not only carry sheets that are filled, but also tons of blank sheets.
Maybe the way I have it set up could inspire you.
I currently carry my Contacts inside my Kendal Mini. Instead of having my contacts arranged in an alphabetical order, which is a terrible 'space eater' due to all those tabs (*gasp*), I have them arranged by their importance in my life:
---CONTACTS TAB HERE----
-boyfriend, father,BF parents house, BF father's +mother's mobile, sister, BF's brother+wife, BF's sister+husband, BF's cousin's house in London, and then mobile for each of the London cousins as well as more cousins in Spain (I'm lucky not to have a large family)
----TAB COMES HERE------
-about 10 contacts here
-about 12 contacts
---TAB COMES HERE-----
Useful Info UK:
-Bank numbers, mobile operator, Utilities, NHS, (this one will grow the longer we live here)
Useful Info Spain:
-Dentist, Doctors, Bank, Vet,
---TAB COMES HERE-----
-Those contacts I don't have as friends, like our landlord, my cousin's half sister in the US (don't even know her), etc etc.
That for me totals a number of 10 sheets including two blank ones. And I am using the Mini format. In the personal format, more info would fit and you'd still keep your sheet number low.
I don't know how you currently have set up this section. So, if you can, just keep it as it is, otherwise try to find a way to slim down the pages.
Here you could also try to slim them down, by storing older ones and just keeping the last 5-10 sheets. I know this is probably not much help, but like the Projects, it also depends on the way you currently use this section.
Looking back at your post, you say "doing future planning in the field with a contractor waiting for me to give him dates."
I was thinking, if you still need to reduce your system, one option for this would be to add in front of your Projects section a couple of pages with only VERY important dates. That way, instead of flipping back and forth in your diary, you can simply look at this list to let them know when is what due. You can either have it organised per month:
3rd: do such and such
4th: blah blah
10th-20th: blah blah
Or per Project:
9/3: blah blah blah
12/20: blah blah blah
9/4: Blah blah blah
10/12 - 10/25: blah blah blah
Either way, it should help you plan those dates better and not look clumsy flipping through your very fat Filo ( joking ;) )
Anyway, this is all my brain could come up with. I hope it is of some use for you, and look forward to reading your posts on your system revamp.