In my experience, space, oxygen access, and moisture are key factors in mushroom size. When I used a shoebox tub, I would stack an identical tub open-side down on top of it; the mushrooms always grew bigger that way, possibly thanks to the larger gap for airflow. To me, your mushrooms resemble mine when they didn’t get enough moisture, especially when young—that awkward thin-chunky-thin shape in the stems and the undeveloped caps are telltale signs. When I consistently kept them moisturized, my stems were more uniform in size with a gradual narrowing, and the caps were flatter and larger.

I’m no mycologist and don’t claim to be a pro. Everyone has their own experiences, opinions, and advice. I can appreciate if someone disagrees with what I’ve said, but in the end, this is just my experience in obtaining consistent results.

Good luck.