From: Jacksonville Fl
Have another look at the base display - you should see the port capacity listed as two numbers separated by a slash:
12,000/9,000 or 9,000/12,000 (I forget which order.)
The larger number is total tonnage (as you said), the smaller number is the largest ship that can be handled. So, in the example above you could have 50 x 5,000 tonne ships and they will all get their turn at the pier - eventually. But, a 10,000 tonne ship will never tie up.
It might get some unloading done via (abstracted) lighters, etc. but that will be very slow and some equipment is simply unable to unload that way. A tanker will probably take forever to unload (if it does).
Hope this helps.
Ah, so that's the magic combination...
So until I find some sub-9K tankers it's back to sending out mini-AKs with fuel... Oh well.
(The more I micromanage, the less I enjoy... sigh!)
Believe it or not, it gets easier. Once you get the hang of what ships fit. There were times I wanted to throw the keyboard out the window. Keep plugging away!