what to bring and some packing philosophy

Why is it that when we pack for a weekend, we generally pack more than when we pack for a week, or two of travel? What is the compelling nature of that large bag for a short period? I think it's a fear of being unprepared, after all, it's just a weekend, and we should be able to bring everything we would need for the time. How many different things can you do in a weekend anyway?
Travel for longer periods is different, we learn to make do with what we have, and if necessary, to purchase those items that we didn't bring. I distinctly recall hiking the Long Trail with my cousin and friend in Vermont, and we would run into AT thru hikers. We would have all this unnecessary gear and tons more food than they did. They had it down to the bare essentials, which for one of them, happened to include a Banjo he had been carrying since Georgia! We were out for five days with large packs, and they were on the trail for months with smaller, lighter packs.

The bag I am bringing, the Arc Teryx Bora 80, is clearly overkill. The problem is that I don't have a medium sized bag. Lots of daypacks, and lots of expedition sized packs in the basement, but nothing in between. I have decided to use this pack, cinched all the way down, so it will be small, but expandable. My packing list is pretty limited, after all, I am going for two months and two days, so flexibility is key, like the AT thru hikers. I am carrying everything I bring everywhere I go, and want to get the most out of each item I am bringing with me, so flexibility is key. Anything left behind during the trip cannot be retrieved. I also have to decide which electronics to bring with me, another large and imposing question to be answered in the next 72 hours.

Current packing list:

1 pair athletic sneakers
1 pair shoes
1 pair flip flops
6 Socks / underwear
1 pair pajamas
3 t-shirts
2 collared shirts
1 long sleeve shirt
1 button down shirt
1 belt
1 pair shorts
1 pair athletic shorts
1 pair dress pants
1 pair light weight pants
1 bathing suit
1 rain jacket
1 baseball cap
1 pair sunglasses

toothbrush and toothpaste
Nail clippers
Razor and shaving cream
soap / shampoo
roll of TP
Travel towel

power adapter(s)
cell phone and charger
digital camera
ipod touch

Passport and other documents
travel books