It is working as designed.
Look into the province that you want to siege. There is a yellow or red number that tells you the supply limit. If it is red it means that the province is out of your line of supply. By establishing a line of supply with over 1000 troops per fortress level for every province in the way to one of the provinces you or one of your allies control, the number turns yellow and is bigger.
That number is the number of troops you can keep before attrition starts to kick in, although is modified by the movement rate of your leader and if it is a port by the blockade of the port with your ships (or ally ships), and in latest betas by fortress level, so the actual supply number should always be a little bigger, even twice as bigger if blockade.
For every level of fortress you must field 5,000 troops. I don't remember mighty but that could be 30,000. More doesn't do any good. You can substitute 1000 troops by 10 cannons, with the adittional advantage that cannons will increase your siege bonus, speeding the siege. Against a mighty fortress without cannons will simply not advance the siege. While the siege bonus increses with 10x, 20x, 30x and 40x number of cannons per fortification level, in practical numbers you should put as many cannons as possible, protected by some infantry, to not incur in cannon attrition (ouch).
