I can vouch for the fact that you can carry too much pack. I do it so I know it can be done.

I grew up in the days of big rucksacks so modern packs seem small to me visually. Luckily they are very light so carrying a bigger pack than I really need doesn't mean carrying a lot of extra weight. It also means I have room for a surprise cookie for each member of the party which usually goes over well.