This waste of space was inevitable.

The female Osmiae, though nearly always larger than the males, present marked differences among one another: some are bigger, some are smaller.

I had to adjust the width of the narrow galleries to Bees of average dimensions.

It may happen therefore that a gallery is too small to admit the large-sized mothers to whom chance allots it.

When the Osmia is unable to enter the tube, obviously she will not colonize it.
She then closes the entrance to this space which she cannot use and does her laying beyond it, in the wide tube.