(Text updated Feb. 15, 2019)
On one YouTube video I watched, about an ultralight hiker’s gear, the first aid kit was a few items in a sandwich bag. An ounce (30 grams), perhaps. And good for a skinned knee, I guess, but not for anything worse. My own first aid kit is built around a serious accident* leading to multiple injuries: large and small cuts (including with arterial or other persistent bleeding), extensive burns or scrapes, a sprain or broken bone, and splinters or thorns driven well into the skin. If you’re prepared for a worst-case scenario you may never face it, but better to carry a little extra weight than to bandage your hiking partner with toilet paper and duct tape.
Non-injury medical problems can be just as urgent. You need to stop diarrhea to keep dehydration from killing you, if it's a hot day with limited water. One full-strength aspirin, chewed, may save the life of someone having a heart attack. You also need to treat minor issues such as backache and heartburn. And you may need to do other things unexpectedly, such as repair clothing.
These days the crush-proof case I use is almost an anachronism; most cases are cloth. At the top I have several things I might want to get to most quickly: a pair of disposable gloves, first aid scissors (including for cutting away clothing), and a clotting sponge and large gauze pads (two 4 by 4 inches, two 3 by 3 inches) for slapping on a badly bleeding wound before applying pressure.
Deeper in the kit my cuts-and-scrapes supplies include a roll of cloth first aid tape, a 2 inch by 4.1 yard roll of gauze, three 4 by 2 inch adhesive bandages, two 4 by 3 inch adhesive bandages, five “owie size” Band-Aids, two adhesive knuckle bandages (good for places besides knuckles!), three small packets of triple antibiotic cream, and a couple of cleansing wipes.
My over-the-counter tablets include six aspirin, six ibuprofen, six acetominophen, six Benadryl, four Pepto-Bismol, four Imodium, four electrolyte, and four antacid. If you’re on prescribed medications, don’t forget those.
To keep blisters from developing on feet: nail clippers and moleskin. Other items: hemostats (for removing tiny spines and other fine work), a tick key, a mirror for self-examination of facial injuries or for signaling, and a tiny sewing kit (thread, a straight needle, a curved needle, and three safety pins).
The next photo shows the kit when it’s closed and secured with a mini-bungee cord. Since I took the photo I substituted two strong rubber bands for the bungee cord. The kit weighs 13 ounces (370 grams). I used duct tape to seal the little breathing holes from the inside, to help keep water out. The case used to belong to my college hiking partner but after I admired a little too openly, he gave it to me. Great guy.
I also carry several first aid items separately from the kit: a 36 inch Sam Splint, an Ace elastic bandage, a WaterJel 4 by 16 inch burn dressing, a Swat-T tourniquet, an Adventure Medical Kits wound cleaning and closing kit, and 100 ml of sterile saline solution. These additional items allow me to deal with sprains and minor fractures of the extremities, burns, deep gashes, and grit in someone's eye.
Admittedly, a kit this extensive is way up on the curve of diminishing returns, in terms of weight and volume versus injuries a hiker likely to see. But if you injure yourself badly out there and turn to a stranger for help, you'll be hoping that the stranger has a kit like this one.
The main thing is, take a decent first aid kit with you every time you step into the boonies. If you're not interested in building a first aid kit from scratch, take a look at Adventure Medical Kits' Ultralight/Watertight series. The ".5" weighs just under 4 ounces (110 grams)and is best for taking care of yourself. The ".7" weights 8 ounces (230 grams) and its contents make sense for serious jaunts.
Last but definitely not least: you need to know how to use the first aid items in your pack. If you haven't already taken a first aid course, make that a priority.
*What sort of "serious accidents" could you experience? Falling down a rocky slope, for one. Or how about a bear attack? You can follow the links to read about real-life examples. Also, the most serious accidents you're likely to see won't be on the hike itself, but on the drive to the trailhead and back. If you come across a car wreck in the boonies, a first aid kit this extensive might make a big difference.