I think that guppy obesity somehow correlates with tank size, current, water changes (= more activity, less nitrogenous stuff in the water, better metabolism), maybe tank companions and the diet itself. Most pet fish are like other pets, quite well fed. As Alisa noted, they do a pretty good job of training us for extra meals. Upon joining Guppylog, I was intrigued by stories of guppies swimming darn close to power filter intakes. (I really feel that adult guppies caught in filters were ailing and in the wild, probably would have been caught in a pike cichlid.) Tank size, water quality including oxygen saturation and current must all play a role in how much exercise they get.

How well fed they are is illustrated by how well they can do while one is away for a week or three of vacation. No one would leave another kind of pet without food.

I've flogged the story (too much) of the dying Ameca splendens females, possibly fed too much animal protein and hurt by oversized fry. Undoubtedly there is some ideal range and combination of foods for guppies.

I think that the manufacturers have probably already done a fair amount of research and work on what they eat, in putting together guppy flakes. It might be fun to ask them if they have a bibliography. I'll bet in the scientific literature, aquaculture literature or even in Houde's book  (you don't see what you aren't looking for) are studies from the field on what guppies eat. Also, wild fish don't eat the same every day. Some days they may eat well, parts of others they may spend just staying out of the way of predators or waiting to drop fry. Obviously their diet itself varies from day to day. Wild guppies are also smaller, though they actually might eat more and burn it off.

As for those two-year-olds, they are oooooold! As any creature gets older, warranties run out and parts just don't work as well. As you know, they are much more vulnerable to bacteria and the possibly resulting dropsy because they are old and may already have fluid retention problems and their resistance to "bugs" may not be what it once was.  As Guppyluvr pointed out, additional weight may just aggravate those things. Of course in nature, a two-year old guppy is probably just about unheard of.

If they are chugging along, having a good time, that's pretty cool and a feather in your hat, DaysSweetDaze. :)