Surface mk grid boxes: why so shallow?

[thevenin]

Recently worked on a job which needed several multiway grid switch positions complete with 2 module dimmers.The 1st box,24way was about 55mm deep,which gives plenty of wiring space for 1.5 cables & dimmers.But strangely when a 12way box was fitted,the depth drops to 40mm,which can be a problem with dimmers & neutral connectors if fitted.Has anyone had the same problem?
Regards

 

[holmslaw]

..

 

[Lectrician]

And if you read the install instructions for the dimmers you will find you are not permitted to over populate the grid and backbox with dimmers.....