Maybe the worms at the bottom are the older worms ergo bigger?
Although to our eyes there seem to be no food, what we don't see is NO fresh food. There's still lots of MO the worms can live on.
Having no fresh food in the surroundings may cause them less eager laying cocoons, I think.

Maybe grow to extent of support of the environment is somewhere in there. Having all that room for just a few worms.

How long have you had them and what kind of size are we talking about in '2 to 3 times' the rest of the colony? I come across a few here and there that are noticeably larger than most of what I see but just figured it is the older worms maturing as I got my first worms around 8 months ago.

I opted for the EH myself as a more suitable worm for the garden though the size for fishing may come in handy some day if I ever have too many I can off them into indentured servitude.

By bigger I don't mean any longer, just fatter. The rest of the colony has always been pretty skinny maybe the width of a bamboo skewer. I've only had them about 8 months myself as well (and I assumed, perhaps incorrectly, that the ones I bought were mature. I've also wondered whether they shipped me EF by mistake).
The bigger ones I came across yesterday looked like proper nightcrawlers but shorter.

Ok, I get you now. I have seen that same thing with some being more like the diameter of a pencil and others as you mention being slim.

There always seems to be a mix of these sizes when I look all the way down to very small worms. My assumption was simply that they were of different levels of maturity.