Little things add up.
- Do you need a set of batteries a day for your GPS or is 3 sets enough for a week.
- Take food out of boxes. A plastic bag is 6 grams. A box is 40 grams. -- about an ounce. Repeat for 21 meals, that's a pound and a half.
- What's in your repair kit? I found last full check I did I had ladderlock buckles incompatible with my present gear. How many safety pins do you really need?
There's lots of lists of this sort of stuff. Some people are fanatic about it. I heard of one guy who took the extra 4" off his boot laces. Another who cut out the liner of his running shorts.
It can go too far. Another guy used a cheap $2 use once special rain jacket. And it rained the whole week. Fortunately for him, I had a small roll of duct tape.
Generalization: For longer trips you need better gear. Sometimes better=lighter. Usually better=tougher.
Keep in mind how long you are going to be miserable if something doesn't work. I bring 2nd pair of footwear with me. Twice I've had to loan my 2nd pair to someone who brought the wrong footwear with him.
If we got normal weather, and I've worn everything in my pack, I figure I've cut it too fine. I should have 1 layer more than I expect to need. What do I do if someone in the party twists an ankle on a rainy windy pass, and we are moving at 1/3 speed the rest of the day.
You can cheap out for low probability events if you have a way to endure it. E.g. I might not bring my rain gear for a trip in Canyonlands, but resign myself to waiting it out huddled under my tent fly.
My repair kit has more stuff in it for a large group than for a small group. Peel and stick mosquito netting to fix holes in tent netting. 1 each of every type of buckle on every pack. (Doing a week with a broken waist buckle isn't fun) A sewing awl. A weekend trip, I won't bother with the roll of duct tape. For a 3 week canoe trip, I brought 5 small rolls of duct tape. Group was 25 kids and 5 adults. (Voyageur canoes) Every paddle, bailer, pack, gear had a stripe of coloured tape on it. Made sorting out the gear at portages infinitely easier.
My first aid kit is somewhat group-size dependent, but more trip-length dependent. I carry enough anti-biotics (broad spectrum ampicillin and check if anyone in your group has allergies) to treat one individual for the length of the trip. Burn dressings for 1 individual for the trip, or to next resupply point.
You find that 'expedition grade' stuff is a lot less fragile, and often somewhat heavier that the super light stuff.
The less you carry, the lighter your load, but also you are less capable of dealing with the unexpected.
"Take care of the pennies then the pounds (British money) will take care of themselves."
"Some are penny-wise but pound-foolish."