IMG has a huge ZOI. They are allowed to pew people in systems with an IMG base, so it's a pretty powerful PVP ID (more so than the BHG ID for instance). GMG is far more limited in terms of ZOI but has some really awesome snubs.
I requested the cargo limit to be put to 4,3k. It was in an attempt to level the playing field between the different mining factions and enable others to draw players in.
Though I'd still prefer a limit of 4.5 k... it'd enable IMG to fly one more transport, the Crane, which was used by them even in vanilla... :p
But anyway...
It only makes sense for IMG to not be flying around 5k transports, considering the areas where IMG works. Those big transports are not really that fitting for these areas. Besides that, yeah, obviously it's about balance as well. Not to mention, if you want to go trading, maybe you wish to rather look at all the house trading factions out there. All have good routes available, some have access to almost all lawful houses even.
Since you can't mine and fly at the same time anyway with a big transport, a good idea would be to pick a (fitting) house trading faction for the transport and have an IMG miner work for your ore.