Looking at a 40 liter backpack, you might think there's no way you can get even a weekend's worth of gear in there! But with a bit of effort, you'll be surprised just how much you can take with you. It's like a backpack version of a clown car. LOL
A basic (and maybe not perfect) example is comparing a pair of jeans to a pair of leggings. Jeans will take up about 4 times the weight and space as a pair of leggings. Now multiply that times the number of clothing items you bring. If your entire wardrobe consists of smaller, lighter clothing, you have already saved a ton of space and weight in your backpack.
If you ever shop for performance clothing, it's not unheard of for the actual physical weight of the item to be listed. If you have a kitchen scale, you can even weigh your clothes to see the difference it will make in your bag!
Also remember that you'll be packing a very efficient capsule wardrobe (see The Process for Creating Your Perfect Minimalist Wardrobe). You don't need many clothing items. Maybe around 10 items total. That's way less than I pack for a even a weekend near home! Plan on doing laundry along the way so you also only need 1 pair of socks and 3-5 undies.
Shoes take up an enormous amount of space. You should not need more than 2 pair of shoes. Whether it's a sandal or hiker, bring a good pair of walking/hiking shoes. Then bring a pair of easy sandals (I usually bring rubber flip flops). When you are moving from place to place, if you wear your walking shoe, you only need room in your bag for your sandals.
Not only do you want to limit the shear number of items you stuff in your pack, but you only want to bring as much as you need. Another important trick is to get rid of all packaging.
Let's use your first aid kit as an example. Say you want Imodium, ibuprofen, tums, a sewing kit, some duct tape for holes in your mosquito net, and bandaids. If you bought all that stuff at the store and put it in a pile, it's going to be quite a lot. But take a packet each of the medications, a small roll of tums instead of a bottle, a hotel sewing kit, only a few inches of duct tape rolled up, and a few loose bandaids and they easily fit into a snack sized ziploc with room to spare!
Same goes for any liquids. Use either travel sized shampoo, body wash, toothpaste, etc. or get refillable travel containers. If you are only going on a week-long trip, you don't even need to fill them all the way. I even bring a small sized hairbrush, tiny loofa, small small small. Only bring as much as you need.
Everything you think of bringing, try to consolidate, minimize, and find smaller, lighter versions.
Lastly, packing cubes and folders help keep you organized. They do add a little weight just because of the actual weight of the cube itself, but they also help save space by grouping and compressing all your gear. Roll your smaller clothing into cubes (you can even check out compression cubes). My favorite is putting almost my entire wardrobe into a packing folder. A month's worth of clothing in one small bundle!