I'm no expert on Chicken Behaviors, I might as
@aart ,
@3KillerBs ,
@21hens-incharge @cmom if maybe they could weigh in.
To my (limited) knowledge, a chicken doesn't normally suddenly start laying small eggs after laying large ones. Yes, its normal for birds to lay a few smallish eggs as their plumbing works the kinks out, and then for egg size to grow in the first weeks to months of their initial laying year - but that's it. Not egg size that suddenly and visibly decreases.
And while I can (and have) linked to studies showing that average egg quantity (of lay), quality (contents), and size increase on a diminishing basis with crude protein intake, those amounts (while statistically measurable with high confidence) are so small the typical flock owner wouldn't notice. Certainly with regard egg size - you need calipers to measure the average difference. Sure, there's some variation w/i birds, but mine that normally lay md-lg sometimes lay a lg. and the XL sometimes lay a jumbo. But not that one of my Comets might suddenly start drooping md-lg eggs after having dropped XLs all year long.
I've no explanation. Wasn't aware it was possible. My Golden Comets still lay huge eggs, they passed 1 yr of age a few months back. My SLW still lay Md to Md-Lg. So do my Brahma.
As to the Marans (I've not owned one), they are famously broody. And their egg size, apart from a dark brown color, is highly variable. Any chance the eggs you are seeing are from her?
Oh, there's plenty of Calcium in that feed while I support the use of free choice oyster shell, and generally oppose the use of "layer" feed, a shortage of calcium isn't causing your Comet not to lay. If a lack of calcium was an issue, you'd much more likely note weak shelled eggs, and even "shell-less" eggs and a decline in your bird's condition before laying stopped.
I'd instead look for signs of early molting, and evidence of vent gleet or other signs of possible reproductive problems.
Then, if I thought it might be disease, I'd post pictures and summon Azygous Eggcessive or Wyorp Rock for additional assistance. Overo Mare is good on lots of these topics too.
Sorry I can't be more help.