My FAKs for hiking and such are usually from Boots or wherever.
They're generally about a quarter of the size of a box of breakfast cereal, but the ones in the Jeep and the truck are approaching the size of a shoe box. In my case that's quite a big box.
I've got a really tiny FAK (Lifeventure) for the motorcycles. That's because it takes up less space but it's a legal requirement in some countries.
I think it would be useless if I actually had a serious motorcycling accident. The one time I offered first aid (to a Spanish biker who'd just dropped his bike on a mountain road in France) the offer was brusquely declined. I think his pride was injured more than his bleeding knee.
The commercially available kits never seem to contain enough micropore tape, nor any surgical alcohol, aspirin, insect bite relief, paracetamol, rubber bands nor a decent knife.
So I add (or carry) those, and one or two other odds and ends. In fact most of the time, apart from the clothing I'm wearing, the additions are all that I carry.
I occasionally do minor surgery on myself with my Victorinox Signature, but I'd be very reluctant to do it on anyone else (members of this forum excepted, obviously).