Crocrosmia

... well, they do need to be reasonably deep, especially so with the bigger varieties such as Lucifer, The stems of Lucifer are quite heavy, so if the bulb is too shallow, then the stem and flower will flop or fall over.

Lotty with bulbs it is usually four times the height of the bulb but as Gary says some bulbs need a little more depth.
Crocosmia once it settles will grow a string of bulb-lets down over like a string of onions and they will produce a thicker patch of flowers. My Jackanapes has out grown its bed and over flowed into the borders and a gravel path the weather has been perfect for it and it a mass of flowers.
Once it goes back I will tackle it and reduce the size which means getting every tiny bulblet out, like mint it needs containing, be careful where you put it.

Frank.

Now I know why my Lucifer always falls over if not staked! Thanks Gary.

They also grow towards the sun, so if in a shady spot they will bend towards sunlight, further bending them!