I think there are a number of options here, but probably not all in one uber cargo module.
Smuggler's Hold would be a small cargo hold which is resistant to scans. Illegal goods moved into this hold wouldn't be detectable by remote scan (or less likely to be detected). These may exist as a "part" of a larger cargo module (64 tonne capacity split 60 normal and 4 hidden), or as a small dedicated module. Obviously filling a T9 with smuggler's holds would fool no one...
Cargo Safe would be another small cargo hold which is armoured and protected against theft.
I don't think that a hold which carries more would swing - as its all suppose to be a containerized system. If anything you'd be trading capacity for capability. Since cargo modules don't add mass, this would likely be the first thing to be added.
Our ships only deal with one-tonne cannisters. I would presume that the bulk transports (invisible megaships) deal with larger units than this. There may be a future in a large-size cargo module which can hold a single bulk transport container - but that would require a lot of background work first.