On the leftovers, which is the more interesting half of your question.
Coming home with a day and a half is extremely common and it has two causes that need different fixes:
Appetite lags effort. For the first couple of days a lot of people eat noticeably less than they planned, and hunger arrives properly around day three. So a flat rate per day over-supplies the start. Packing lighter for days one and two and heavier for the back half matches this and saves real weight.
Packing meals rather than weight. A meal is an unbounded unit and it is always generous. Weighing turns it into a number you can decide about.
The structural fix, though, is not in the packing at all: plan around resupply. Carrying five days of food is a choice, and on most routes it is avoidable. Half a day off-trail to a shop removes two kilos from your back for the harder half of the walk, and the time cost is usually less than the time cost of walking slower under the weight.
When there genuinely is no resupply, then the density arithmetic above is the lever, and being deliberate about days one and two is the rest of it.
One thing worth doing after every trip while it is fresh: write down what you had left and what you were sick of. Two trips of that and your list becomes accurate in a way no general advice can be, because appetite on trail is quite personal.