Loading Rucksacks

I've just been trying to stuff 4 nights backpacking gear
(food included) into a 35 litre rucksack (old Berghaus
Dart). It was bulging at the seems a bit but I just about
managed it. Unfortunately it wasn't particularly comfortable
to carry however I adjusted the straps and the same load + a
bit more food besides in my usual 45litre sack (Lowe Alpine
Walkabout) felt considerably lighter despite the fact it was
actually about 700g heavier. I guess this is because it
holds the load closer to your back which it manages despite
the space wasting air cooled back system. So I guess that's
end of that little experiment. Now I've just got to put it
all back away again till next week. Don't want to leave my
sleeping bag stuffed in the bottom and I fancy the chocolate
bars that are in there somewhere too.