When I first discovered this I thought it was a bug and therefore reported it as such but was told it wasn’t.
Almost all items in the game can be dragged into your cargo hold in the fitting tool to see how much it can fit. Except Standard Small, Medium and Large containers.
These are containers that are used to get that extra bit of cargo space necessary to fit more stuff. Oftentimes cap boosters. So, it’s very useful to be able to simulate how a container would work inside your cargo hold.