I've also experienced variation in leaf sizes when I've grown oregano. Maybe leaves left to grow for longer - growth at the tips is always smaller to begin with, IMPE.

I have to wonder. Are the leaves going to continue to get bigger?

Leaves around an inch are quite normal for oregano. That's actually the size they should be and they should be a nice medium green color. The old leaves are a bit on the yellow side in your pic.

Rodney

Mine get big leaves regularly. Especially if I fertilize (which is rare, but sometimes if I have a little left over from another application, I'll add to the herbs). Many of my perennial herbs get big leaves poking around amongst the smaller leaves - oregano, lavender, sage, mint, even the rosemary will have some huge needles/leaves. It does look odd when it happens.

Doesn't seem to affect flavor much. But other than basil or dill, I tend to cook with these or dry them for winter use, rather than use raw - maybe if you use raw, the big leaves will taste different?

I just find it curious that oregano can't seem to decide how big it's leaves ought to be. Never seen that effect with my mint and rosemary.

I have common and golden oregano, and both growings have larger leaves as well as smaller ones. The common does it a bit more predictably than the golden.

I agree with most the others: I have a very old Oregano plant and has a variety of leaf sizes depending how young the smaller ones are.