On the question of the size of later casts, I think there is some exagerration going on here, at least for the "middle years" (i.e. Seymour and the 2nd Foggy period). The only character I felt was sometimes being forced into episodes was Eli, as he would show up for a short scene every episode for awhile. Beyond that, a number of the supporting characters might only show up for one or two scenes (the ladies coffee morning is in many cases the only time you see them, for example), so its not like the episodes were generally dominated by the secondary characters. There were also episodes where at least some of the secondary characters weren't shown at all.
I generally find the size of the later casts feels natural, and in some ways reflects the audiences interest in the bigger "world" the characters live in, which was not just limited to what the core trio were doing. In one of the books on the show I've read they refer to this idea as "Summerwineland," which reflects that although the show took place in a real location, the people making it realized it was not the real world that they were presenting in the episodes. These 70 and 80 year olds were doing things that were not likely, technology was slow to show up, etc.
That's a good point about technology, in Spring Smiler Smiler asked if he could just ring Nora and Foggy telling she's not on the phone, in 1993 its hard to believe someone was not on the phone. ( I'm sure I'm about to be shot down now but generally speaking most people had a phone line by then)