I do 20 day trips with very long duty days. A 22" bag holds everything for winter and summer. The synthetic t-shirts are great. You get them at Ross or even thrift stores (new tags still attached) for a few dollars. A couple of nylon running /swim shorts work great as underwear (and don't scare anyone when it is bunk time), swimming, exercise and sleepwear. All this stuff is lightweight, quick drying, wrinkle free. Sweater (thin fleece), pair of jeans (heaviest item in my bag), 1 short sleeved dress shirt that matches my jeans or uniform pants (business casual). Pants with zip-off legs (use them as shorts or long pants for walking / hiking on lay-overs). A pair of sneakers or hiking boots depending on season, and that's where socks and shorts go into. If it doesn't compress, fill it up! And a pair of Keen sandals for pool, beach, or warm climate wear. Two uniform shirts -really can do with one since that's the first thing to come of once on the plane, advantage of flying boxes. 1 pair of pants (company pays laundry after a couple of days). As an active swimmer I use Speedo Performance jammers swimwear: this material is light weight, dries very quick and holds up very well when used in chlorine- or salt water or when put away wet. I also use them as compression shorts for exercise / hiking. Previous Nike shorts of same price fell apart after 3 month, these last years. A small backpack, inflatable neck pillow and airline blanket complement this. All together the whole bag weighs less than 30 lbs.
My flight bag contains laptop, flash light, noise cancelling headset, documents, light rain jacket and pants (I walk a lot and don't want to be pinned down by weather, also handy for preflights in the pouring rain or blowing snow) and other nick-nacks that are handy on the flight deck.
When travelling as a passenger (which equals to about 150k miles a year) and I have to check my bag (never check my flight bag off course) I use my backpack to make an emergency kit in case my bag gets lost: sandals, hiking boots, swimshorts, sweater, t-shirt, zip-off pants, pillow. I always wear my uniform pants and shoes, and either the uniform shirt or the dress shirt (the uniform shirt will be in the backpack in that case). So now if my bag gets lost and starts chasing me around the world I have my uniform, I can go to the pool (while my underwear, socks and uniform are drying after being washed), go for a walk or go for a bite to eat. The rain jacket is enough to keep me warm and dry.
For the sink laundry I carry a couple of hotel shampoo bottles filled with detergent and softener. The bar of soap does wonders for stains. I carry several wooden cloth pins, and everything gets pinned to the a/c unit and in an hour (sometimes a few hours) everything is dry. Wash your stuff when you walk into your hotel room, and when you wake up everything is ready to go.
One question you have to ask yourself: do you really need a new t-shirt, underwear or socks everyday? If you sweat a lot, yes. If you don't and only sit in your seat the whole day then there is a way to reduce the amount of stuff you carry with you. Washing your underwear and socks when you get to your room can reduce these items to 2 pair each, no kidding. Same for uniform shirt.
