Well I'm on annual leave at the moment and considering Elliott's response in the Freighter Design thread I've had a quick look at this.
In my current game all freighters are designed to have more cargo bays than the default. My small freighter has as many cargo bays as a medium, my medium has as many as a large, and large is extra large by a similar proportion. So I've considered a couple of times during old saves of my current game (all around mid-game when I'm doing the majority of expansion and upgrading) what proportion of total freighters are carrying cargo above the default.
Small: 6-12% (i.e. this is the proportion of the time the capacity of a Medium is used)
Medium: 2-8% (i.e. this is the proportion of the time the capacity of a Large is used)
Large: 1-4% (i.e. this is the proportion of the time the capacity of an Extra Large is used)
Keep in mind this is the proportion of the total. Some will be refuelling, getting retrofitted, inactive, or flying to a source of resource.
This needs to be balanced against the extra maintenance cost for the private sector. That said, I never find the private sector short on cash even on Extreme, I've funded these larger Freighters for many games.
So it would appear there is a case for giving a Small Freighter as many cargo bays as a Medium Freighter as a minimum. There might be a case to simply make all Freighter designs essentially a Large Freighter with cargo capacity. However, adding bays to a Large Freighter, as Elliott alludes in the thread linked, seems to have little value.
With mining ships, for those that are active, it frankly looks like all of them are mining beyond the default cargo size ... as others have suggested I also give them a lot of cargo bays.
Since the game is designed so that private ships have a numeric cap ... optimising their designs to move / mine more or faster per ship is valuable if you can afford it ...
< Message edited by Icemania -- 8/8/2013 2:39:20 AM >