Eating The Elderly

Something weird seems to happen to people as they pass around sixty. Up until sixty, or so, eating with your family has one defining feature - there's always a lot of food!
It occurs here that I may need to remind people that I, of course, am English, and it may strike Americans as weird that one could have less than too much food at any meal, so I am going to have to point out that food in the UK is approximately four times as expensive as it is in the States. After, of course, you've factored in a variety of issues such as the exchange rate and taxation etc etc.
Part of the issue may be simply that I am a lot larger, so perhaps there is some truth to the idea that all food is perhaps smaller, but after polling people this seems to be true - after people hit about sixty they completely lose the ability to discern a difference between how much they eat, and how much other people eat.
I completely understand that once one gets to a certain age one's appetite drops away sharply, that's normal, I expect it to happen to me one day, assuming of course I live to sixty. What I hate is that when this happens, the senior types seem to simply lose the ability to judge. It has got to the stage where whenever I and my other half are invited to eat 'a sumptuous feast' such as Christmas Lunch or a Birthday meal, rather than assume we're going to be well fed, we make sure we have a large meal at home before hand - this achieves two things;
A - We end up properly fed.
B - If there are other people at the meal, then we're able to have smaller portions, thereby allowing other guests to eat more, and thereby not literally starve to death.

I guess the thing I most hate about this is I don't really get how this blind spot can exist - it seems both universal and peculiar. I mean, in any other area you adapt to the people around you. If you're knitting socks you wouldn't knit the same size socks for a child as you would for an adult. You tell clean jokes at funerals and dirty jokes at weddings, well, at least if you're me, but in any case, in pretty much every environment people can tell the difference between themselves and other people, so why is it when you eat with the Seniors, they say things like 'What do you mean you'd like another potato? You've already had a potato!' or bring out the main course for eight people that consists of a single chicken and a half dozen sprouts? I'm 6'5" and big with it, I need more food than half a sandwich, dammit! Surely that's intrinsically obvious?!